Mum and I were talking about this the other day actually- she has a lady at work with a teenage daughter whos father has recently had another baby- the mum insists that the new baby is her step-sister and goes nuts if the teenager refers to the bubs as her sister or half sister (never mind the fact the baby IS her half sister)
She also told mum that I shouldn't refer to my brothers as brothers as they are my step brothers (again they are half brothers!!)
My brothers are my brothers, I don't care that we have differnt dads, we couldn't be closer if we had the same dad. I get really annoyed when people say oh so they aren't really your brothers? Um yes they are!