Is there a graceful way out?

I had a nasty breakup with ex gf, and her sister is convinced I abused her and told me point blank that she is going to beat me up. She is some kind of black belt in martial arts and I study BJJ. This is a no win situation, she obviously has the capacity to hurt or even seriously injure me, and vise versa. I've got no plans to fight a woman even if she is capable...if she wins, I've had my kicked by a girl half my size, in public...and if I win, I would be considered a bully and a coward. Any ideas on difussing situation?