Uh boy...this sounds exactly like my older sister. Only she is way older, like in her 60's now. She has and will always be a bully. There. I said it. Absolutely no learning curve there. I'm pretty sure she is set in stone at this point.
If one did venture to "discuss" or confront the weird &/or inappropriate behavior, she feigns not to know of what you speak...
I cut all ties. She did this to herself because no matter how many chances she was given, she always reverts back to bad behavior.
I had to cut a wide berth around her life and mine. All of my friends and acquaintances get it & are cordial but not friendly as a self protective mechanism. They have all seen her turn vicious then pretend it didn't happen. She co-opted friends only to try to turn them against me, over & over again. Facebook became her lurking grounds, then she'd use what info she'd glean as weapons of destruction out of the blue. Please beware! I hope your deal isn't as bad as mine was.
I still don't trust her as far as I can throw her. I got a Christmas email this year & Promptly deleted without opening. There is no trust that she has any good intent. She was fishing. Sad but true.
Sorry to hear that you too have this koo-koo behavior in your world... At least you know you're not alone...