So, I'm the mother of a singleton which means most of DD's friends have brothers and/or sisters. Often I'll invite one of her friends over or out to an activity and the parents of the friend will either send along one (or two or three) siblings or like today, say, "I'm sure you won't mind taking her brother along too" when we called a friend to go to the cinema. And yes, I do mind. I don't feel like having both, he's not DD's mate, the sister is, and although they're all friends of our family, sometimes I just don't want your extras.
Maybe I'm missing something or just not understanding because we have one, but is the onus on me to accept siblings or can I just say "Ah, no thanks just the one" in polite terms?