Is it normal to date his ex

My brother was dating this girl,but they had a misunderstanding and I tired to help whiles helping this girl told me she was not going back to him so me I stostopped 2 days later she has weather I had a feeling for be be honest I said yes that's when she also told me she do so the last time a realized we were dating.she is older than me so things started getting naughty,her sister came to slove and things went well. Yesterday she started talking with my brother now telling him things she told me and now my brother hates me what should i do please help.