What?! You can't keep your child away from her half-siblings.  That will surely cause the demise of your relationship, and then your daughter will go to her father's house and see those half-siblings anyway. If you didn't like his kids, you shouldn't have married him and had another child with him.

Unfortunately, kids today are exposed to a lot of stuff and think they know way more than they do,  with so much time spent on the Internet. Figure out how to make peace with the kids (though I don't blame you for not wanting to go the ex's house on Xmas morning).

Well, the good news is that by the time your daughter is old enough to understand, the skids will be much older and presumably will have a better handle on their own sexuality.  I agree with others that you are not going to be able to prevent your daughter from spending time with her half siblings, especially if your partner wants the three of them to have a relationship.  That doesn't mean you need to spend Christmas with his ex-wife, though.  There is no reason that your partner can't spend part of Christmas with you and your daughter, and part of Christmas with his other children, if he insists on seeing them on Christmas Day.  Put your foot down now or you will be expected to spend holidays with the ex every year.  If you don't want to, tell your partner that you are not willing to accompany him to play happy family with his ex and that you expect him to spend most of the holiday with you and your daughter.  What does his CO say about parenting time over the holidays?

My SO's ex wanted him to spend Christmas with her and the kids at her parents' house with all of her siblings and their kids.  She thought the kids should experience the holiday with both parents.  My SO was fine with that, but I most certainly was not.   If she wanted the kids to experience holidays with both parents, she should not have left him.  This was a hill to die on for me.  So last year we took the kids on vacation the week before Christmas and celebrated the holiday with them then, and they were with their mother for Christmas.  SO briefly facetimed with them on Christmas day, but that was it.  This year it's SO's turn, so we're taking them on vacation the week of Christmas and their mom can celebrate either before or after that.  I want to start our own traditions, not spend my holidays doing what BM's family has always done.  BM here is normal, she's a good mother, she has no interest in getting back with my SO and she has included me in holiday invitations and encourages her kids' relationship with me, but I still do not want to spend Christmas with her.
 

Your partner needs to split his time between his families, you don't have to.  Do not set this precedent.  Do not entertain this nightmare in the making.  

Children of divorce can learn to adapt to the split if they are encouraged and see adults acting accordingly.  When Dad can't make up his mind who his heart belongs to, they get mixed messages and it sounds like these kids need stability and rocks to lean on. Not a wishy washy Dad.  

If it was so important to be there for the big moments, he should have never gotten divorced.  

With EX his wife.  You should split your time,  Get SK in afternoon.  You Open gifts  with Bio in morning.   You can not start doing thing like this, all every thing will include ex BM