I personally think you did a good thing by telling your brother about it. I believe in honesty being the best policy. It would have been better if your ex-gf would have been honest with your brother and told him the truth. She was trying to start a relationship with a lie and that does not work and even if it does, it is a big secret that she would have to live with for the rest of her life. It shouldn't be a big deal to you or anybody if everyone has moved on with their life. Now your brother can decide and if he really loves the girl then I don't think so her past should matter.