My friend...

[Mysterious F.]

...found out his girlfriend was related to him.

She broke up with him about five seconds after they found out during lunch, and it was hilarious how he was just like "But we're related by law, it doesn't really matter!" and was o the verge of tears.

Naturally, us being teenagers, just about everyone in high school knew by the end of the day.

Also, last period they have Chemistry class together, and they're part of a four-way lab group, so... yeah...

[Keaton]

The family that plays together stays together.

[Darth Wyndisis]

Whoa, that sucks.  How long had they been together?  Hopefully they weren't too closely related.

[darkphantomime]

Remember kids! It's not incest if it's not a blood relative!

[Uximadesk]

But there's still that little taboo there.
Also, a friend told me about how two friends who were together ended up being stepsiblings, so it was very awkward after that end they eventually broke up.

[Pyrgusfinn]

a similar situation occured with some of my friends too. It was strange.

[DW]

Me and my girlfriend are apparently fourth cousins, but nobody can figure out or remember how.