SUBREDDIT: r/tifu
TITLE: TIFU by thinking a brother and sister were dating.
POST: This happened just last night, right before the midnight screening of Star Wars: The Force Awakens. Me and a few mates had taken our seats in the cinema and a couple of guys and a girl we knew were sitting below us, with two of them being brother and sister (I didn't know they were siblings of course). 

Me being the little shit I am, I make a joke to the friend who isn't one of the siblings saying "third wheeling there I see?" and then he says very loudly in response. "THAT'S HIS SISTER."

My friends burst into laughter, both the brother and his sister look at me very fucking weirdly and I knew that I had fucked up very, very badly. I also might add that his sister was extremely hot, making the situation a whole lot worse...
TL;DR: 
assumed a brother and sister were dating (didn't know they were siblings), made joke to guys friend about him third wheeling. yells out that his friend and the girl are siblings and made myself look like a jackass.