Not Your Father's Advice

A guy decides to get married.

Well, Dad, I’ve decided! I’m going to get married.
Who are you going to marry, son?
Lisa, Mike’s daughter.
Oh, don’t marry her, son. You know… Ah, fine! I’ll admit it — I sinned in my youth — in short, she’s your sister.
Well then, maybe I’ll marry Emily, the neighbor girl.
No, son, I messed up back in the day… she’s your sister too.
Then how about Mandy from the next block*?***
Don’t take her either. Same story — she’s also your sister.

At this point, Mom can’t take it anymore and yells from the kitchen:
Don’t listen to that idiot! Marry whoever you want — he’s not your father anyway!