Why when people have siblings but only share one parent they call them their half brother or sister?????

I mean which half is your sister and which is not, are they a half person? No so why can't you just say my brother or sister????

I'm only asking because my kids have a sister from their dad but they just say sister, but when other people address it they would say half sibling, and that kinda annoys me, I've heard a lot of people address their siblings that way also, I've always wondered about that.