I would ask your SIL to please keep everything you've told her in confidence. Beyond that, there's nothing you can do, and worrying won't help.
Even if your STBX found out everything you said to your SIL, I doubt it would carry any weight in terms of the divorce proceedings. Judges don't care about that sort of drama. It's normally very dry, just about who makes how much for the purposes of alimony and child support, and also about full disclosure of assets so that things are divided fairly.
The process is so stressful. Hang in there.