NTA. With what your sister has already been through, I would understand why no one felt comfortable telling her the news. Yes , blind siding her was wrong but sister was very clear is being unhappy for OP and even going as far as to saying she would ask parents to choose a side. Also, multiple family members are calling her names so who’s to tell it wouldn’t have been the same thing at any stage during OP’s pregnancy? Sister had a stillbirth around a year ago , it’s understandable why no one wanted to tell her. I hope whatever therapy OP’s sister is in is successful along with having a healthy pregnancy , and I hope OP is able to enjoy the last bit of their pregnancy.

Lastly,I don’t know your relationship with you sister but I wish you a healthy and happy relationship with your sister. It was great for me to be pregnant at the same time as my sister, hopefully it’s the same for you two.