I'm thinking of saying that my wife's sister could technically repopulate the Earth with her brother in the event of the apocalypse
Why would you say something that odd?
It was just a thought that popped in my head.
Some thoughts should be kept to yourself. It is weird and strange to think incest would be a good idea to repopulate the planet.
I'm sure she wouldn't mind her sister having sex with her brother. It's the fair thing to do.
There are things you shouldn't say and keep to yourself. This is one of them.