@Raella50
YANBU!
Sorry but I am in the same band of people who say she doesn't get to treat you like crap because she cannot conceive.
It's not YOUR fault if someone else - (a family member or friend) is unable to not have a child, and the family taking the sister's side is a disgrace frankly.
I have known (like a few other posters) several people who couldn't conceive, and they literally ignored by 2 kids. Like they weren't even there. I obviously got rid of them out of my life after several years, but yeah, I had that shit too. I also had a so-called friend who could not get a man to stay with her for more than 4-5 months, who never married, and who never had kids, and every time I saw her, (when I was with DH and the kids) she ignored them, DH and my 2 kids. She also talked over me when I mentioned them.
Couldn't bear it that her formerly chubby, plain, 'boyfriendless til she was 20' friend, had got married and had kids and SHE hadn't.
I feel sorry for ANYone who can't have children, or who is alone, but it's rude and nasty to take it out on those who have what THEY want.
As a pp said, how long do you have to keep pretending your child doesn't exist, because she doesn't care to acknowledge your baby because she can't have one?
I would be moving away and distancing myself from your family if I were you. They sound fucking awful, and you deserve better treatment.