Can't win either way in THIER eyes? Sounds about right.

Sounds like it's time to start looking out for you since no one else is. Start doing what makes YOU happy and what YOU need to do. If they don't like it, too bad. Time to disengage.

Yea being a step parent can feel like a no win situation. Can you be gone during the visits at a friends place, family member, or a hotel which is costly? Spare yourself knowing "not my kid, not my problem". Sadly I'm not hearing from you that you are receiving support from your husband. You may be the SM but its up to the birth parents to find a babysitter when they can't do it themselves.

If you can't get away during their visits, its time for you to enjoy a nice long bath, watch a good movie, & pamper yourself..the magic of disengaging

You've joined a family who resents you and doesn't want you there. Tell SO to go back to his ex, who he obviously adores since he does her bidding, and find a childless man. This will either wake him up or be the end of your relationship, but you have to take the leap if you want to be put first.

Ahhh...the sweet, sweet life of a disengaged sm. Let dh know you're stepping back, not being a bitch. It's all on him now. If you have an issue, tell daddy and have him deal with it. It's a great feeling, trust me!

Disengaging is hard, I will not pretend it's not. My own maternal instincts, I raised three decent people already, kept kicking in and I seriously had to hold myself back from opening my mouth when I saw DH 'pandering' to them all.
Let DH know that, with regard to the sleepover, you really had no prior consultation so you are really not prepared to deal with it. If he asks what you need you can list a whole bunch of 'entertainment' items, and a SPA day for YOU that will cost him a fair bit! Sucker if he goes and buys them all, right?
Tell him by all means WE can take care of the girls, and stay up late to make sure they do not get too rowdy or, worse yet, sneak out to see boys! I sincerely hope you NEVER have to endure what we have had here.
Let me tell you about SD here. A friend came to spend the night, the two of them sneaked out to a party. DH went up with medicine (SD had two STD infections) for SD and discovered them both gone. Cops were called, the other girl's mother had to come and get her and this other girl was all three wise monkeys rolled into one! SD was already on probation, she was arrested for violation at her next review and given 30 days for a dirty drug screen. To a manipulative, scheming, teenage girl like her it was just an occupational hazard that she got 30 days! She really does not care what the impact of what she does on other people...so why should I care?
I would also suggest to your DH that his daughter really is there to visit with HIM! I can also tell you that I spent more time with SKids here than both their parents combined, I stopped with all that too! I referred them all (three of them) back to their dad every time they wanted something from me.
It's worth bearing in mind that both you and DH could have different thresholds for what you can tolerate, mention that to him? Let him know that you do not have the same biological connection to SD and close bonding just doesn't just happen overnight, not just seeing her once every two weeks, and on DH's time really. Suggest to him that there be much more consultation with YOU before they BOTH expect YOU to comply with their wishes? Do all of this as calmly as you can, with a smile, and be sure to let him know that if your wishes are totally disregarded then you cannot participate...right?
It's hard, it can work out though! I do feel more liberated now that I do not have the every day stress of SKids...they survived to be 19 (twins...boy and girl) and moved out. SS came back, and DH is doing everything for him except wipe his ass! It's really, REALLY, hard to stand back and witness all of this...DH is on Chemo therapy right now too...last night SS cooked and left one hell of a mess in the sink, dishes and food, and the stove was nasty too. I pointed it out to DH, since he deals with SKids and next thing I know DH is cleaning it all up? DH stayed up to do SS's laundry too!
dh also cleaned out the bird cage...I heard him struggling and huffing and puffing and I said 'get SS to help you!' DH refused to listen to me. Can you even imagine how hard it was not to put in my ten cents on that one? That's part of what disengaging is about, it's letting them get on with all their crap and NOT letting it bother you, or more importantly not letting them know it bothers you. Yesterday I broached the subject of SS paying rent, saying we needed to come to an amount that is reasonable. Yep, you guessed right, DH forestalled me! I am not doing without anything, it seriously doesn't impact me but we did discuss it weeks ago and DH said SS would be contributing to the household.
Stay as sweet as you are! I wish you well and I sincerely hope you NEVER have to deal with anything as severe as some of us on here have experienced.

I'm not sure if others do so or not but I have 2 step kids, one toxic & the other adores me..I recently refused to further see the toxic step kid until I feel other wise but want contact with the other one. Is it right or wrong- well I listened to others here & decided that enough is enough when it comes to the BS. Now what I do have going for me, is a supportive husband which can make it or break it. It was only a matter of time before I was going to cave until others here gave me permission to disengage from my toxic SD. If DH could not understand that.. well it would of been his loss in the end.

Honestly only you can answer for yourself as to how much you want to/can tolerate.