I'm sorry you're feeling so hurt and alone at the moment. It is hard when we think we're doing OK and others, for some reason, give us the impression they don't think we're all right.
However, sometimes we are mistaken about their motivations. You say you are the only one who wasn't asked to help. Perhaps she didn't know you wanted to help. Perhaps she thought it would tax your reserves too much FOR YOU and that you'd be upset with her if she asked you.
Here's one thing I know for sure: Being left out of that is not a reason to off yourself...It just feels lousy. Perhaps it is a reason to tell your sister-in-law, "If there's anything I can do to help, please ask."
If it turns out that she is biased against you, then her opinion is not going to be supportive or of help to you, in which case, why bother being hurt by her...prove you're useful in some other venue and be successful IN SPITE of her opinion, hun.
Please be safe. Thinking of you.