Hi ~ I'm sorry this is all happening. This "uncle" definitely has some problems and needs therapy badly.
My friend, you said that this girl seemed to have a personality disorder -- well do you really think that she'd believe you if you told her? Since she views him as a "father figure" and tells him everything, I seriously doubt that she'd believe anything negative that you tell her about him. She's totally on his side.
I have to agree with your husband. Stay out of it, and let her find out on her own. She will at some point. She'll find out something that he's said that she told him in confidence. It's only a matter of time.
When dealing with family matters like this, it's just best to sit back and be neutral. That way, you don't get into trouble.
God bless and PLEASE take care. Hugs, Lee