Don't worry, you'll understand someday." (Pat her condescendingly on the head.)
A cheat sheet on how to handle sticky situations, from drunk uncles to bitter siblings.
Uncomfortable situation: The perpetually bitter sister (saddled with four kids) asks if being single is the reason you've "put on some weight."
You respond with: "Thanks for keeping such a close eye on me, but it's rather the opposite: I've been having so much fun as a single person, I'd forgotten what it's like to worry about every single pound. It's kind of liberating!"
Lesson: Kill them with kindness. Any bitterness or socially inappropriate behavior will soon be deflected by your smart, mature replies. Good luck!
Spoiler ahead: Agree with me or not, but Michelle Williams and Ryan Gosling are the worst couple ever and I was pretty happy about them getting divorced.