Maybe , and I'm only partially joking here, you could start a "boyfriend jar" --every time she starts up the subject-a quarter in the jar from your sister!!

I guess strategy isn't a helpful word. I'm thinking of any action that may surprise your sister enough to catch herself. For example if she goes on and on and you just get silent--how long would it be before she catches herself by picking up on your silence?

If she has to talk about the boyfriend so much, maybe the relationship really isn't that great.

I know how difficult family can be--I hope she doesn't drive you crazy!!