Half sister

A couple had been married for many years, and their son had gotten old enough to date. One day the boy brought a girl over to diner. The mother was thrilled with her son's choice and couldn't wait for the wedding. However, the father was upset and, eventually, the boy asked, “Dad, why don't you seem happy with her. Mom likes her a lot.”
The father explained, “No son, there's nothing wrong with the girl. It's just that I cheated on your mother a long time ago, and the girl you've been dating is my daughter by that woman.”

So the boy dumped her and found himself another girl. Again, he brought her home to the mother's delight, but the father again told him this girl was actually his half-sister. The boy lost his temper and told his mother what his father had said.

Furious, the mother shouted, “Don't listen to him, sweetheart! He isn't even your father!”