So I have a similar situation. My husband helped raised his exes two daughters and then had my now ss. Ss lives with us full time and we take the girls pretty much every other weekend (they live with their dad) because we like having them around. Some weekends my son goes there and we get a date weekend which is nice too! My husband loves those girls and now so do I. The girls call me and my husband their step parents. We go to their school functions and birthdays and have them for holidays, I help with homework. My parents get them gifts as well and they have been to their house too. They were also in our wedding and their dad was invited too. We get along great with their dad- BM is barely in the picture but we have had times with all of us together too. They are a part of the family to me. In my opinion my stepson is my kid, but they are his siblings and a part of his family so they are a part of my family too. The kids loving hanging out together. And it was important to my husband- he bonded with them and I get it. If anything ever happened to me and him I would be crushed to lose my son I helped raise for so long.

I can see why it would be weird or something you wouldn't want to take on, she isn't your responsibility-but at the same time she's just a child. She'll be in your sks life forever so there's no harm being kind to her too. You don't have to take care of her or have her over if you don't want to- even though I can see why she would want to. Start small and just get a small gift and show kindness to your sks sibling. You don't need to be her extra babysitter or second mom but a friendly face of her siblings step mom is always a good thing?

Just a thought as well but sometimes I like having all the kids because they play with each other and keep each other occupied which gives me a breather sometimes lol