The brother/son is important because it's usually about the sister/daughter, and it represents someone who is not the sister/daughter. Like, it could mean someone outside of the family is like part of the family. It could also be a competition with imagination that there are step-siblings.
My reaction...? It's just a calculation and also a real question if you want to proceed forward/onward.
😊
Oh, and the brother/son is still important, to me too, just so you know.