PIGA.....I thought I was being too suspicious but I saw the red flag when it went up. This would not be the first case where a woman betrayed her own sister and went after the BIL. I have personally known of three cases! Isn't that awful? It hurts so much worse b/c it is your sister.
The fact that there has been some jealousy between the two of you makes the case of her going after your H even stronger. I hardly think they are just talking about yours & his M stitch. They may have started out that way.....but that is not her interest right now! Even if she wasn't out to do damage--or to see if she could get him interested in her--or for whatever reason she had.....it could have become an EA with them talking that much together. He is vulnerable and she is taking advantage of the situation.
I don't know if you will take to this idea, but it's just a thought of how you "could" turn around and "use" her tattling to him. Here's what I'm thinking....Don't talk to her any more unless you decide to tell her (later) that you've done a lot of thinking and have decided that you were going to let him go. He's not worth it...and you've decided that you want to be free and single again. Keep it short and simple and don't fall into any of her traps of answering too many of her questions.
If you can tell her that with a convincing tone, then she will probably break her neck to tell him (before she stops and considers what she's really doing)... and then he will put his focus on you instead of her. The thought of you wanting to be free again....well, it will have his mind filled with all kinds of new concerns!
But, you will have to wait a few days to do this b/c I doubt she would believe your story right now. Then after you tell her that....don't discuss your M or H to her any more.
It's just a thought based on knowing how some people's minds work, but you know what you can endure and what you can't. I just know that people usually want what they can't have...and if your H thought you were no longer available to him, then he would be interested. Also, the thought of him with your sister is somewhat "taboo" for them and that adds to their excitement. So, maybe it would knock the props out.
(((PIGA))) I'm so sorry.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!