Coming from someone who has been married 3 times now (God I hate admitting that), DH and I won't ever share finances. That was something we both agreed on early into dating. We have both been hardcore screwed over. However, we give freely and openly to each other. We both have each other's login access but we don't have each other's names jointly on anything but our mortgage. 

I am a big fan of his/hers/ours accounts if you want to co-mingle. Each keep your own account but also have a household account for all the bills and groceries. That way you are equally contributing to your life but you still keep your independence financially. 

Combining finances can be tricky.... especially when one party or the other may have different spending habits.. or financial obligations.

I would recommend deciding how you want to split your living expenses and shared financial goal savings.. then you each would have your own accounts checking/savings for your personal discretionary spending and then have an account that you both contribute to for your joint spending.. and maybe even a savings account as well.

I have used USAA for over 30 years for my banking and insurance needs.  I will say that I am not sure that the insurance is the cheapest I could find.. but they have always had good customer service.  Banking has never been an issue.. well, I did have one bad experience dealing with a rep over my dad's account.. but it was kind of a special situation.. I understand why it was difficult.  I like them because you can easily deposit remotely.. and can do lots online like pay bills.. transfer funds with non-usaa accounts etc... rates are decent.

But, there are many "national" banks that offer similar services.. I might also recommend having a relationship with your local credit union too.. so you have a walk in location if you ever need those kinds of services.

It's not wise to combine finances with someone who has more assets or liabilities than you do. If you two were going to start from nothing and grow from nothing, with no baggage, I'd likely still advise against it but could see where the benefit would be.

Here's the problem: until SD ages out, there will ALWAYS be someone who has an interest and stake in your BF's finances. If you combine your income and God forbid something were to happen to him, it'll be a financial/legal nightmare with BM since SD may still be entitled to some of your BF's estate. That doesn't just include physical property, but any assets - like life insurance, savings accounts, and even money in his checking. You'll have to prove what is yours so it doesn't get entrusted to SD.

Futuro's solution is a good one. You have a BM who would do anything to get access to your BF's money, so keep yours away from his. If you all have the same bank, it will be super easy for you both to transfer funds into and out of a joint checking account for expenses and a joint savings account for trips, big expenses, emergency fund, etc.

IF you are hell-bent on having a joint account, make sure that you BOTH keep a separate savings account that you're each depositing money into. That is your PERSONAL money. Trust me, there will be things you want to purchase that you don't want BF to know about (like gifts, nothing nefarious), and things you won't want to help pay for related to SD and BM. It's hard to imagine what that might be NOW, but it can (and likely will) happen.

It's risky to combine finances, but if you're going to do it, you need to make sure you have your own nest egg (that includes your own HSA/FSA for healthcare expenses, 401(k) or other retirement account, other investments, and a savings account with cash) that cannot be touched by someone else. And no matter what you do with finances, your BF needs to create a living will that explicitly states what money goes to who, AND he needs to get life insurance for the amount of CS he'd owe if he were to die today that goes to SD (or a separate account with that amount in it). That will help protect you and your share of assets if something were to happen to him.

Remember, financial planning isn't just short-term. You have to think about worst case scenario for the next 3, 5, 10, 15, 20, and 25+ years. 

Lastly, combined finances are rooted in women not being able to open their own bank accounts and keep their own finances. As women become more equal, and employers stop offering joint financial insentives (like spousal insurance), you're going to find that less and less of your finances will be co-mingled anyway. Realistically, the only finances you'll have combined will be your checking/savings account, home equity, and mutual debts. Just about everything else finance-wise will be separate.

I do not have shared finances with my DH (beyond one joint account for household expenses, which I handle), for two reasons.  First, he is a complete and total financial incompetent, by his own admission.  If he has it, he spends it.  He doesn't understand debt or interest.  He doesn't remember to pay things on time.  He makes stupid financial decisions and has crappy credit.  I'm far from a financial genius, but I can handle the basics and I'm smart enough to know when to seek advice, and I have family members who are financially savvy and give me great advice.  Second, I came into the marriage with assets (he had none to speak of) and he came into the marriage with debts (albeit not major) and obligations to his two kids.  I didn't ask for a prenup, but I kept my accounts and my home in my own name and nothing is commingled.

In my circumstances, combining finances would be foolish.  It doesn't matter that my parents (who are equally financially adept) have joint finances or that I went into my marriage thinking it would be forever.  In your circumstance, where you're roughly financial equals (except for SD) and you're on the same page financially, it might be OK.  Just think about whether you're OK with paying child support if it's ever ordered, or paying for gifts for SD, college for SD, expenses for SD, out of your funds.  Because that can happen.  If it gets out of hand, you can always separate finances in the future.  Also think about how things are set up (joint with right of survivorship, for example) to protect yourself if something happens to your BF.   

I don't think combining or not combining finances has much to do with how your relationship proceeds - you can have a great marriage with separate finances or a crappy marriage with joint finances.  Just make sure you've considered the contingencies and that you're comfortable with everything.  And be open minded - folks on here are giving you some great advice and things to think about.  

Alright, since you're going to have joint finances, here is what I would do differently if I ever combined finances again:

1.) Keep a savings account that I only have access to, and put small amounts of money into it each month. That would be my "oh sh*t" fund.

2.) Keep a centralized ledger to put all expenses and receipts into. Needs to be somewhere that both of you have equal access to. 

3.) Set up a joint system for how you manage money. Whether you reconcile monthly or every two weeks, you both need to sit down, look at what's in the bank and what's in the ledger. If there are discrepancies, then sort them out then.

4.) Don't nitpick what the other person buys. If you like daily Starbucks and that's a habit you won't give up, then build it into your budget and tell your BF it's going to be a daily expense so long as it's affordable.

5.) Set a limit on how much an item needs to be before having to talk about a purchase. This goes back to the "don't nitpick every item" piece. You each should be allowed to buy a pair of shoes without the other flipping out. 

6.) In addition to setting that limit mentioned above, set a weekly allowance that you each have for your own fun. Build it into the budget. That is an expense that you know is going out every month. Anything not spent should remain for that person as part of their spending money, not thrown back into the joint fund.

One of the big killers with joint finances is nitpicking every dollar saved and spent. Set it up so you're not paying attention to that. Whether it's each of you having separate credit cards you pay off every month or having a set limit that you BOTH can stick to for fun money, just figure out some way that you're not criticizing a Starbucks or Monster Energy habit. Joint finances, I've found, require a lot more communication than split finances, so you'll need to make sure you both speak the same financial language. DH and I don't, which further strengthens our resolve to remain separated financially. He tallies by the month; I go biweekly. He accounts for "third paychecks" the month he gets them; I account for mine based on when bills are due on my biweekly schedule. We can't find common ground on making joint finances work with that wildly different mindset, so we just mostly check in to make sure bills are all paid, CCs are paid off, and our agreed-upon joint savings is where it should be.

I'm sure there are more tips/hindsight I could provide if I thought on it more.

OH, final thought: joint finances can become separate finances if the need arises. Don't cling to joint finances if practicality says it would make communication better, tensions less, or whatever because joint finances are your ideal. Your ideal should be a happy, strong marriage. If joint finances go against that, then don't have joint finances. 

My DH and I combined our finances when we got married. Neither of us were rolling in money or assets so it made it easy to combine. I balance the monthly budget and pay the monthly bills, while DH takes care of state and fed taxes and larger stuff like refinancing the house or insurance stuff. I keep a spreadsheet on our computer of the monthly budget and we can both update it as needed. This is what works for us.

Keep in mind that by keeping a separate account in your name you're also protecting him. As long as he is in the military CS will come directly out of his pay. No problems there. He leaves to become a civilian and BM decides to pursue CS increase/ area all accounts in his name will be frozen. That's a bad position to be in when you have bills to pay and other children to support. There are many potential financial issues when you are dealing with OP and SKs. 

A lot of this is a legacy of patriarcial rules that have slowly been overturned.  Until relatively recently, it wasn't legal for women to have their own bank accounts.

France, 1881: France grants women the right to own bank accounts; five years later, the right is extended to married women, who are allowed to open accounts without their husbands’ permission. The US does not follow suit until the 196os, and the UK lags until 1975

1974: Equal Credit Opportunity Act passes in the US. Until then, banks required single, widowed or divorced women to bring a man along to cosign any credit application, regardless of their income. They would also discount the value of those wages when considering how much credit to grant, by as much as 50%.

US, 1981: The last vestiges of a husband being able to keep a wife in the dark (at least legally) vanish, thanks to Kirchberg v Feenstra. A husband is told he doesn’t have the right to unilaterally take out a second mortgage on property held jointly with his wife.