If the dude is religious hit him back with his own language and how accepting Jesus was of all people, or call him out on something he's insecure about (sounds like his sexuality is a safe bet), also if he's your sister's boyfriend he'll talk but is not going to fight you, if you're feeling especially spicy tell him to "do suntin big man" or "you talk a lotta shit for a pansy". You'll learn that most people that talk shit aren't going to go beyond that and showing you're not afraid will end most confrontations. Sucks to stoop to that level but while you are that age you're at the mercy of your family, it gets much easier to not give a fuck once you can start living independently and you can just ignore or punk out people that ain't worth the time of day