Does she often lie in this kind of way? If so, maybe for important things like assets you can include another person in the discussion or discuss via email or something so you have a record? That way your son or someone else can judge the situation for themselves. Hopefully if interactions with your sister make you ill you don't have to interact too much. I had a friend who I felt was rather controlling, but at least not lying or manipulative. It seemed out of genuine concern, but it still messed with my head. I am working on setting healthy boundaries so it doesn't happen again with her or anyone else.