I NEED HELP ASAP PLEASE has nothing to do with periods

So I'm dating this guy and I dated his brother in the past (horrible I already know) and me and this guy have been dating for a week and at first I had feelings for him and now the more I see his brother the more I don't have feelings for him and the stronger my feelings for his brother is idk how to break up with him cause I know I need to cause it's horrible to be dating someone and not have feelings for him please help