Well...it's just my personal opinion. But from my perspective this is all between your sister & her husband. She has told you to back off & leave her alone. My opinion is that's what you need to do. You wrote you're overwhelmed, highly stressed & upset about this. This is not about you except to the extent you're making it about you. My recommendation is just leave it alone. And if you simply can't let it go on your own, consider talking through it with your therapist, if you have one, or seeing one if you don't.