P.W.
Lizbeth,
I don't know from what you told us what reality is, but I can tell you this.....
there was no reason for your Mom or your sisters friend to share those things with you. They were of no benefit to you. Don't we all sometimes vent about our relatives a little? It may not be right, but we do it and we don't necessarily want to share those feelings with the person we are upset with.......we are just talking out loud. Sometimes our feelings are more about ourselves than the other person. So, first of all I would tell your Mom and your sisters friend not to tell you these things anymore. They are just hurtful.
Next, if you have concerns about your sister, how about taking her to lunch and telling her. "I'm worried about you. You seem upset lately and I want to help. What can I do?"
Then accept that you can only help her if she wants it. Love her the way she is, but protect yourself too. Her journey is just that...... hers.
I hope it gets better soon. She may be jealous, but you can't do anything about that...... and she might not be able to either.