As long as you don't let them near your own box, it's ok. When they fsck your parents pc up, they get blamed, not you. On the other hand, if they need something you're really good at, and you know they will be happy as heaven when you do it, do it yourself. That way your creditlevel rises, or at least stays the same, and that of your brothers at best doesn't change (I'm evil, I know.. >:) )