I think best is to just move on with your life. Distance yourself from your brother. Block his sister-in-law so that even if she unlocks you, she can’t ever contact you again.
I’m wondering if this “thing on her phone which tracks where her ex i s turns out to be a tracking device she has loaded onto his phone. Hmm. That’s stalking and I don’t think it’s legal.
Just keep them out of your life.