if you truely care about your brother and can afford to buy your own rig, why not let him suffer with the peice of junk for the time being while you build him one of his own? out of your own money? consider it a birthday or christmas gift, i bet he'd love you for years for it.
also you earn brownies with your parents for being mature and responsible (as long as you discuss it with them first, play the present twist)
dont destroy thiers, they may have files on there that are critical to be kept, if you destroy it you may loose your rig.
im sure that if your bro has gone this long with the junker then a few more months wont hurt. even if you just build him a low spec one im sure he'd appreciate it.
if they wont let him have his own, build a rig to replace your parents, and call it a gift to the family. even more brownies lol and no one looses really. you'll be given a breath of space and be shown more respect by them and no one can get angry over it.