I'm sorry to hear about your situation - a toxic one such as many of us have suffered, with the BM stoking the fires of hate and PAS due to resentment or in some cases wishing she hadn't split with DH.   I'm afraid it's often an SM's lot to be blamed, but that doesn't mean you must take it lying down. There is often a honeymoon period initally where everyone gets on - we saw this with NPD BM and the Walking Wallet (her DH) - for the first few years the sun shone out of his backside for the SDs - now they absolutely loathe him. 

 The best thing you can do is not get involved AT ALL with BM - don't allow her any contact with you by any means - your DH should handle it all.  Plus you and DH need to establish robust boundaries and standards of behaviour for all the kids and absolutely insist on these being adhered to.  Often DHs will not do this because they don't want to alienate their little snowflakes - but the Disney Dad approach never works well in the long term.  Kids actually aren't comfortable with too much power.   Your SKIDs don't have to have a particular sort of relationship with you or your daughter, but they must be required to treat everyone with respect.  This is your DH's job, and if he isn't up to it, you have a major problem. 

I don’t say this often, but personally I’d seriously think about leaving this situation.  It all comes down to your DH.  For some odd reason, the dad/ DH is rarely mentioned in any and all SM vs. BM or SM vs. SKs situations.  Yet, he is the main one who will determine his wife’s or SO’s acceptance into the household.  Manipulative, controlling BM (or older SK) and weaker, enabling DH = step hell.

So, bring your DH into the mix and ask yourself what has he (or has he not) been doing for you to help further transition you’all into one household?  Has he been doing what a lot of our DHs on Steptalk do, and that is pretending there is no big problem, putting his head in the sand, saying he doesn’t want to be put in the middle, etc.?  It is his divorce.  He should be in the middle of the fallout from his own divorce, and not you.  And that includes, dealing with both his ex- and his children appropriately and setting appropriate boundaries with them.  As dad, he is the one with the authority.  You have little to none with his kids.  He should not be leaving it up to you to manage not only your own children and the new child, but also manage his children too!?  No!!  Flat out No!

So, first of all, you and him need to get on the same page.  I don’t care if he works odd hours or isn’t home that much or deserves his rest, etc.  He needs to be managing his ex- and his kids.  He needs to treat you like the wife you are and treat his children like children (no matter what age) and his ex- like what she is, an ex-.

If you’ve been in this relationship enough to know that that is not going to happen in a 100+ years, then you need to think of yourself, your children and including your ours baby.  Don’t think about him and his kids at all, because he’s already made the choice that he and his kids are separate from you and yours and can’t be bothered.  Seriously think about what can you do to make this work best for you and yours.  If that means leaving a hostile household, so be it.  Talk with a professional about this, if need be.  If you have to settle for sloppy seconds, then that means your children will have to too. And  I’m pretty sure you don’t want that.