G.B.
She sounds like she is having a hard time. I am glad you expounded on the previous letter. This makes better sense to me.
She may be totally depressed too. She is going through all sorts of life changes and needs to grow some new skills. She is in a transition stage which is the hardest.
Since you are close to your in-laws I think you should sit down with them and have a frank discussion. They need to be aware she feels left out, unwanted, and even unloved by her parents and probably best friend her brother. She has no one except her son and he is an adult who is on the brink of starting his own new life too.
So she is facing an empty nest in the next few years most likely.
No telling how her ex treated her emotionally and fed into this mind set. He could be behind the whole thing. A loved ones words can change us both inside and out and forever. Whether for good or bad.
She is facing life alone, on her own, having to decide to pay bills, drive herself to her or there, all that goes with becoming single again. She may be one of these women who thrive on being a spouse, mother, and homemaker.
I think that the family should pull together and make some observations about how she is acting when she is with each of the family members individually. If she is acting the same with everyone and saying the same thing to all or if she is playing everyone against the other. If she is saying one thing to you but another thing to the others then it can only help everyone to deal with her as a unit if everyone knows what is happening with each other....that sounds confusing, cohesiveness is not just a way for a team to do the best job, it is a good way for a family to address some actions that a family member is doing to cause distress to some of the family members.
Going home crying from family get together's is just a sad thing. No wonder she is not feeling better at them. She has thought loops going on all the time and they need to be broken. If she is feeling left out everyone needs to make it a point to include her in all conversations and activities. Even a few women doing the dishes is a bonding time for those women. They talk and grow closer. It is not a time to leave someone out. Even if she hates doing dishes she could come in and visit.