Does she work?  Is she planning on quitting work? Does she get child support from the father?  why is supporting them your problem?

Have your two talked about a budger?  Is she expecting you to pay half?

It is not your job to support her kids.  They have two parents to support them.  You should thoroughly discuss finances BEFORE you get married.  Many people going into a marriage with someone who has kids from a prior relationship keep their finances separate, although many people combine finances also.  Among the things you should discuss is how household expenses will be split.  Will each of you contribute a percentage of income or will she pay 80% of expenses, since 80% of the people in the household are her or hers?  What about cars, and activities, and college?  Does she have any expectation that you'll be ponying up funds for that?  I'm not sure why, if she is capable of supporting her family herself through her job and child support, you would need to give up your Starbucks and craft beer.  Hash it all out before the wedding or it could be a major bone of contention afterwards.  If she's expecting you to pick up more of the expenses than you're expecting, or if she expects you to support her kids, it's better to find out beforehand so you go into marriage with your eyes wide open.

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There’s no reason you can’t keep enjoying craft beer & Starbucks just because you’re marrying someone with kids. She had those kids, not you. It’s not and never will be your responsibility or obligation to support them. If she’s expecting you to contribute to her children in unreasonable ways, that’s a conversation you need to have before you get married. You pay your share of mortgage, groceries & bills etc, and she pays for herself & her kids share. Anything you do for your skids is a bonus, and should not be the expectation. They’ve got 2 parents to care for them already.

I have 2 kids, he has 3, (only 2 at home now). I make $80k, he makes $300k+. We split bills 50/50, previously 60/40. Married 10 years, nothing combined. Keep it separate. 

Coming from a girl who loves craft beer and Starbucks herself...LOL

Keep it separate. My hubs and I do things very fairly and it works for us.

We rent, we pay the rent per bedroom. So in our 4 bedroom home, I pay for my Son's room, my daughters room and 1/2 the master. He pays for his son's room and 1/2 the master. Younger Stepson is only with us like 6 weeks a year so we don't factor him in the equation. I pay most of the groceries and utilities. He pays the cell phone plan (Him, Older SS, Me are on it) and cable- He likes a ridiculous package and I cut the cord years before we met and just did internet, Netflix and Hulu- so that is on him. Since I pay groceries, he pays when we go out most of the time. I will offer when I can or split things up in some way. 

He makes substantially more than me. About 30K a year, I am not destitute by any means, but his money is his. My kids are my responsibility just like his are his. 

Remember, your Skids have 2 parents to support them. If you do something to help. that is kind of you, but should never be expected.  

ALSO--- SUPER IMPORTANT--- Keep dating after you are married. Go to those breweries together, take trips, go out without the kids. Don't stop doing the fun stuff just because it is easy to get wrapped up in daily life with 3 kids in the house.

I honestly just assumed it would be my job as her partner to help support the children 

Why would you assume that? Because you are a man? Because you will be married? She made those kids, with their father, and it is up to the two of them to support them. She currently gets child support and works to support them, that should not change, they are HER responsibility. Sure, you can pay for things for them when you want but that is entirely up to you. You will (should) still have your money to buy craft beer and Starbucks.

Absolutely have separate accounts. Nothing breeds resentment more than money in blended families. You need to know that you are not being used. You may not feel that that is her intention, but when more of your money is supporting her and her kids than her money is, you will feel it.

If she has a problem with this logic (and it IS logical), well, then one has to wonder why. Your financial burden should not increase.

My SD in now 28 and my husband and I STILL have separate accounts.

But now as we get closer to marriage, I have been given more responsibilities.

Again, her children, financially or otherwise, are not your responsibility. Period. You are not a babysitter, a play date, a chauffeur, or any other thing that you don't want to be. It is not her place to give you responsibilities when it comes to her children. You have the right to make sure they are respectful in your home but don't let her try to get you to take on the disciplinary role, instead of her.

I was single (obviously) and childless when I started dating my DH. I never wanted the responsibility of kids (so never had any). That did not change when I got with DH. His daughter, his responsibility. Not that I never did anything for her, or with her, and I bought her things, but it was at my discretion.

These things need to be discussed BEFORE marriage, it will not just work itself out.

It's not your job to support her kids, and please don't fall into that.  Split payment of bills based on income and the fact that she is supporting 4 people and you are only supporting yourself. 

Allllllll of the above = +++++++++

"But now as we get closer to marriage, I have been given more responsibilities."

Other than the financial part that others have so wisely commented on above, what types of responsibilities do you feel you are now starting to take on that you did not have before you started interacting with the kids?

Just no. You do NOT automatically acquire those children to support upon completion of marriage to this woman. They have parents and you are not one of them. Of course your house, your rules kind of thing, but your wife and their father are responsible for supporting them. So hold onto your craft beer and starbucks!

Finances are always a good/great discussion to have, before as well as after marriage. Retirement plans, budgeting, all that.

My DH supported me fully the first 2 years of our relationship. As I started making more money over time, I took on more of the bills and household expenses.

NOW, 5 years into our relationship and our first year married, we make equal amounts on paper, but he does sidework, that brings his income above mine. That being said, we split all household expenses equally, only because currently he has 50/50 with SD13 only. Ive stopped paying her phone bill, but dont mind buying her things or taking her out places, shes sweet and appreciative. His money is HIS money. If we want to go out, sometimes we split bill, some times one of us will treat.

Now that our housing costs are going up due to home purchase, we are looking to budget tighter. However, DH knows that it is HIS responsibility to take care of Munchkin SD13, so he will provide car, clothes, anything else. He pays child support, alimony, and has debts, so thats another reason we keep our finaces separated.

I agree with others, pre-nup to keep your retirement, also other assets.