Clearly this is a sticky situation. I'm not sure what I would do in your position, but I generally err on the side of inclusion. First off, I get that these kids are older and are sharing some inappropriate info, but I do think their existence in your family is an excellent way to teach diversity and how to overcome adversity. That being said, I think I would sometimes be frustrated by their behavior and/or "oversharing". However, they are family, and these kids do need to learn about how families work and that families love, so as hard as it is, I think maybe you should invite her...
I also had the though of maybe talking with your sister and suggesting maybe she come for some cake, presents and not sleep over, but that's kind of harsh too. I don't know...this is a tough decision. I think you have to really think about how you would want your daughter to be treated in a similar situation and trust your instincts, maybe the two thoughts are in disagreement, but you have to do what you think is right, whatever the decision might be.