lol, your little brother wants to look cut, and not lift a house... you can both!.. im naturally thin, so im year round thin, six pack showing all the time... its the typical 15 year old.. they just want to look good and get the girls (who doesnt)..
i dont know what you could do about your parents... as long as they allow you to do it than i guess it is okay and wont effect you too much as long as they dont prohibit you from doing anything, and taking a good supplement (i.e protein and creatine..etc).
Also, good job on changing your brothers physique..