I'm really sorry to hear you feel like you're being pushed away and sometimes the proverbial punching bag. I understand she's really important to you and it sounds really awful to feel like you're being shut out by someone who means so much.
I have not been in the same situation before, but I can hear your pain as you try to help your friend manage hers. I am not sure what the best action here will be, but have you spoken to her older brother and sister in law about this? It sounds like the four of you were very close.
Otherwise, it sounds like you are trying to do all that you can do - be there for her, support her, but give her enough space to live her life the way she thinks she needs to.
In my own experience, I've not had this happen but I am still friends with my ex and we've had to create an emotional distance between us, which is somewhat similar. I found it really hard to learn to adjust to the new dynamic - it was almost as if we had to recreate the friendship based on new commonalities. Perhaps that is what may start to happen while your group learns to adjust to the new dynamic where you're not the same single misfits. It sounds like it could be a bit of an emotionally tough period for you all, so please don't forget to look after yourself while you are also trying to be there for your friend.