I just had a little trial of what could be the life as a stepmother and I can tell you that you found the right place and I wish you had find it sooner.

The problem is not you is the only thing I can say with my brief experience....

Your feelings are valid. You had a honeymoon period with the SKs now real life is setting in.  My first experience as a stepmother was wonderful so unfortunately, I had unrealistic expectations the second time around.  I truly believe my first experience was a fluke, because I have quickly learned that being a SM is the most thankless job in the world. You are in a position where you dont have the same rights as the parents, and at times you are completely powerless, but are expected to do everything a parent does without any gratitude,  thanks or appreciation. I clearly see the difference and am more than happy to point it out to SO every chance I get, because I am also a bio mom. He always says well your kid doesn't do ....insert bad behavior.  Then I respond that's because I discipline my kid!!!

He is (non) parenting out of guilt due to the failure of his holy first family.  Sadly this syndrome is more common than leaves in the summertime.  

Counseling, disengagement or the best solution, divorce and vow to never get involved with a partner with a failed first family.

You say in your biog that you have anxiety and depression and ask what is wrong with you? If you didn't have these before then they are being totally caused by your circumstances at the moment.  Depression is often anger turned inwards against yourself - your DH is putting you last priority and being a total Disney Dad, aiming to please his little princesses so that they will carry on coming over.  You are very well aware your DH is a useless parent, and yet you ask what is wrong with you - answer nothing - but a LOT is wrong with him, his parenting and his husbanding!   Many of us on this site have gone through exactly what you are going through now - I can't tell you how common it is.  An all too brief honeymoon period, and then things degenerate quickly.  To be honest, you do not need a therapist, you need a different life. 

You need to rethink your current life.  Is this what you really want?

In the meantime, you'll feel less like the unpaid maid/nanny if you disengage.  Just stop doing anything for the skids.  Your husband takes care of everything - cooking, cleaning, etc.  You don't watch the skids.  You do nothing!  Take a bit step back.  If you have no input or agency then you don't contribute.

Also, just ignore the 13 year old.  When she is awful, mean, etc - give her a blank stare and move on.  Put her in coventry when she misbehaves.   Pretend you can't hear anything nasty, avoid her and act like she doesn't exist.

Then spend your energy working on yourself and whether this is the right life for you.  

Your husband doesn't want you all to act like A family, where everyone gets to form their own relationships and traditions. He wants to you to ASSIMILATE to HIS family, which means you conform to his parenting style and fall in line with their traditions and expectations.

This is a marriage killer because it creates a team of them versus you instead of you and your DH forming the foundation of the family that the girls can grow from. SD13 is already acting like a stubborn weed because she's growing through the cracks in the foundation from your DH's failed relationship with BM and his failing relationship with you.

Walking on eggshells is the absolute wrong thing to do. You have equal say in your household, and you need to hold your DH accountable to his contribution to you and the family. He does not have more say just because he has more responsibility. If your marriage were baseball, he's not the coach while you are a player. You're both players - you as pitcher and him as catcher. The team only works if each person takes their individual positions seriously, and each position serves a different function on the team so it works efficiently.

I think the first and most important thing to tackle with your DH is your timeline for kids. DH and I waited, more due to finances, and I regret that. We're running in to some fertility issues, and this is a crappy time for that. So, you need to set a definitive timeline with DH.

I always recommend before walking into these conversations to give the other side a few days to think about it before discussing, and not wavering from that even if they say "let's just talk now" because emotions can get the better of people:

"DH, I'd like to set aside some alone time Saturday afternoon to talk about trying for our baby. If Saturday doesn't work, Sunday is open for me, too. I'd like to look at a timeline and talk about any measures that we may want to take to help us."

If he tries to tell you that he wants you all to "be a family" before you start and keeps a date ambiguous:

"DH, this does not work for me to have an ambiguous timeline to start trying for children. I am okay with waiting for a bit, but I need a definitive start date."

If he persists on the family thing:

"DH, you allowing SD to choose when and how she can participate in this family makes is incredibly difficult to form the kind of family bond you're looking for. Additionally, you have not asked my opinion on what it means for all of us to be a family, nor laid out a roadmap for what 'being a family' means to you. The ambiguity of that statement sets an unfair burden on me and your children to try and meet expectations that we don't know and may not want to meet. You and I are team players and should be setting family goals and definitions together. However, that's not what we are talking about today. Let's table that discussion for next Saturday, if that works for you."

Never blame his kids because he'll throw up defenses. Keep it focused on him - his parenting, his expectations, how his actions impact you and those around him, etc.

If after these conversations about timeline and the expectations of family don't yield a date by which you and he will start trying for kids (because, IMO, it should be easy enough to nail down a timeframe less ambiguous as "when we've had time to meld as a family"), you may need to consider if this is the relationship you actually want. I'm not saying act on it right away, but definitely be paying attention over the next few months to see if your DH is making any strides toward meeting his own goal of family-togetherness because, if he isn't working toward his own goal, he likely isn't going to work toward your mutually-agreed-upon one.

I also recommend therapy to deal with your feelings about your SKs in relation to not having bios of your own, and with any anxiety/frustrations you have related to your SKs and steplife. Find a therapists that specializes in step families, though. Those who don't tend to offer poor advice about the step family dynamic and can push a "Brady Bunch or Bust" therapy plan that does more harm than good.

I think what lieutenant dad posted is right on, great advice and I wish I had read that in the beginning of my married life.  OP, there is nothing wrong with you, you are experiencing feelings that I think anyone would face when you are joining an established family and feel like you have no voice.  You feel like the enemy because you are looking for order, but your spouse just wants his kids to "be happy", regardless of how anything may impact you.  It is so hard, and it's also hard for others to understand unless they have been in it so you feel like you don't have anyone to talk to.  I don't really have anything to add to the above, other than I found counseling helpful - somewhat for myself as a way to release my feelings, but mostly when my DH and I went together.  Fortunately we were both open to it and willing to use what we learned. I mainly wanted to chime in to let you know you are not alone.  I have found a lot of benefit being able to vent here and reading other people's situations, and I hope you do too!

Custody Order.

This is the agreement between the bio parents that MUST be followed to the letter, that details the minor children's visitation with BOTH parents.

Read on these boards, these 2 words are used OFTEN.

Your DH needs to have one, if he doesnt already. The 13-year old is not mature enough to make decisions as important as when she will visit. Plus this puts your DH under pressure to "engage and entertain" her, be only the fun parent, with no rules or restrictions. This is not a healthy way to conduct the relationship - it gives the child too much authority and power over the parent.

As to your wanting a child. Hmmmm. I would get the ducks in a row FIRST. See how he conducts himself. You already know there are parenting issues, and those will not go away by having your own child.

If things progressively get worse with how your DH is treating you as his wife, and you have a child with him, you will:

a. be stuck in a miserable marriage for the sake of the new baby, or 

b. be stuck with a toxic bio-father that you will be forced to co-parent with.

So - my input is to first focus on getting the custody order in place and have them followed. No more child making visitation decisions. If things improve in that direction, then start the discussions on goals and timelines (as detailed by Lieutenant)