Why do people roast their siblings?

My best friend and her younger brother roast the hell outta each other. I can tell their hurt but they sucked it up. Why can't people just be nice and sweet?
I don't like when people roast or make fun of each other. My aunts and uncles roast reach other. It pisses me off. Some say to me, "Shhh. Dawn (me)! Don't ruin the fun. You know we don't mean what we said." One said, "Come on. Have a little fun."