Date my brorher girlfriend and now fall in love with her

My brother was away for 7 month his girlfriend was home alone I ask her if she want to go to the New York cars show after I take her for a drink then we come back to brooklyn I send her home and a taxi from there we start texting on each other then find our self going out on dates then we made love on we bout falling in love he back now and I miss her so much is it normal ?