Dad joke?
Billy Bob goes up to his dad and says, "Dad, I think I'm going to marry Betty Lou." His dad says, "You better not - she might be your sister."
A few months later, he says, "I've decided to marry Shirley." Again, Pa says, "She might be your sister, too."
Six months go by, and Billy Bob says, "I've decided to marry Ellie Jo." Again, Pa says, "'Nope - she might be your sister, too."
Totally disgusted, Billy Bob is sitting on the front porch when Ma comes out and asks, "What;s wrong?"
Billy Bob says, "I wanted to marry Betty Lou, Shirley, or Ellie Jo, but Pa says they might be my sister!"
Ma laughs and says, "You go ahead and marry any one of them - Pa ain't your dad!!!"