I don’t know if this will make you feel any better but my brother, only sibling is estranged from me and I won’t go into details why and at the same time, even though thankfully my parents have funds aside for caregiving (which I mentally, emotionally, and physically cannot do anymore) my brother will make excuses not to help while I’ll have to take care of a lot of the arrangements. I have 2, and my daughter and son will also have nothing in common with each other. My son has special needs (severe ADHD and autism) and my daughter can’t relate to him. He also has his friends he enjoys being with at his group home and he has no bond with his sister. What’s my point? There is no guarantee that a sibling for your daughter would help in any way. It may do the opposite. I’m starting to think that close sibling relationships are the exception and not the rule, but I could be wrong on that. I hear of more stories of those who don’t get along with their siblings than do.