Even if he is related, their relationship no longer has anything to do with being brother/sister, or father/daughter... that boy outside has something else on his mind other than family reunion. Could be he was on her trail when she showed up and, he may even turn out to be the father of her kids. Don't encourage him. His 'harem' may even be following him, as well. And you will never chase stray cats away by throwing food at them! Don't feed them anything even though they looks so thin.

He will make a pest of himself, spray around your home, and maybe even your front door. Your male cats will become upset with him, too, even if they're neutered, as long as they can see him outside, let alone smell him.

I don't recall if anyone has said anything about the dangers of all the nasty cat diseases that strays can introduce into your family cats.