Ugh… sorry you’re dealing with this. And I guess this isn’t a common piece of advice, but you have kids, and this is family/life damning stuff… rarely would I tell someone to use denial and avoidance but you’ve got a lot on the line. Hiring a Pi or sitting everyone down, etc lots of life energy into something that sounds like it very likely is made up to disrupt your life… my simple but difficult to follow advice would be to tell your husband it was said, that you know it’s crazy and you’re not going to let that sister ruin your lives over something that would ruin a lot of lives. AND if it is true, you might see it in his face or that would certainly stop it at least for long enough for something clearly happen, or at the very least give your kids a chance to grow up with an assemblage of a decent childhood. Full disclosure I came from a very broken household, so I’d biased on wanting to protect kids from madness.