This sounds like the premise of an adult film, but sometimes truth is stranger than (adult) fiction.
A dad going by the throwaway username throwawayQuestion667 claimed on Reddit's "Am I The A**hole?" forum that his son is engaged to his stepdaughter, and he wants assurance that he's not a jerk for refusing to pay for the wedding.
When I married my wife, my son was 9 and my stepdaughter was 14. They grew up together and we all got along wonderfully.
My son is now 23 and both him and my stepdaughter have told us they're engaged. We had no idea they were even dating in any form.
I'm feeling very uncomfortable with this. I don't know when this relationship started, and I'm disgusted by the age difference implications.