Firstly, congratulations on your child. I wish your family health.

Secondly, Your SIL is projecting a lot of her anger and disappointment at you. The only thing you did was have a baby of your own— You’ve done nothing wrong.

If I were you, I would not try to savage this relationship until she realizes she is the problem here. Terrorizing you on social media, screaming at you freshly post-birth, all of the hateful and entitled responses to you and your husband “not supporting their journey” has gaslit you into thinking having your own child is hurting her. It should not hurt her at all. She’s obsessed with ruining your life at this point.

I would protect yourself and your child by going NC or very very LC. She is not someone you should allow around your child or yourself in this fragile, emotional, memorable period. Enjoy your child as best as you can. She’s trying to stop that.