One day a girl brings home her boyfriend to meet her father. She tells her father that she wants to marry the young man. After discussing the relationship for a while, the father tells the daughter that she cannot marry the boy because he is her half brother.
Over the course of the next couple of years, this same dilemma occurs five more times. Each time the girl brought a boy home to meet the father, the father revealed that the boy was her half brother.
After a while, the girl starts to get very angry. She storms into the kitchen and confronts the mother. “Mom, what the hell have you been doing all your life? Dad’s been going around town and having kids with the ladies in the city and now there are five boys that I’ve loved very much and cannot marry them because they are my half brothers!”
Her mom smiles slyly and says, “Don’t worry darling, you can marry any one of them. He isn’t really your father.”