Should I delete my brother's ex-girlfriend from my facebook?

My older brother's girlfriend added me on facebook when they were going out, but they broke up 2 years ago. After they broke up she was acting a little stalker-ish, but she's been fine for awhile now and has a new boyfriend. My brother got engaged recently, and I don't think she knows. I'm in an awkward spot because she's always been nice to me, and I don't want to be rude by deleting her. But I also don't think I should stay friends with her on facebook forever. This may sound dumb to some of you, but do you have any suggestions?