I really enjoyed your OP image, so I'm going to respond, albeit with a very short reply.
My brother is a psychopath, too. So is my father.
Stay away from her.
I know it sounds impossible if you're living in the same house as her. But I find it very easy to avoid my brother. I simply live my life pretending he is not there. I do not ever initiate any conversation with him apart from maybe telling him mum has din dins ready for us once a day. I simply nod and grin as he tells me about all the drugs he's doing and the women he's fucking and the things he's shoplifted. When he would tell my embarrassing secrets to my friends or mother, I would simply laugh along with them or shrug.
He finally picked up that I have no interest in his life and he slowly lost interest in mine too. As will your relative in due time.
Your little blackmail ploy will utterly unfaze her, I don't care how powerful you think your secret weapon is. She will not give a fuck. Not even if it has the potential to get her in prison.