You didn't mentioned if you meant as a brother and sister relationship or as dating. If you are thinking of dating then you have to consider this: If your father had your brother with another woman then he has your father's genes in him and YOU SHOULD NOT be dating. If your father (or mother) married someone else and your step-brother was the product of the former marriage then you are not genetically attached. You are very young (I know you don't like to hear that) but don't be in such a hurry to grow up. For now remain in a good relationship with your step-brother and there is certainly nothing wrong going to music concerts or movies.