I hate to put the interrogation spotlight on you when you've had a bad night already, but noneedtoknow zeroed in on a vital issue here: You say you "snapped" at your sister's boyfriend, who made some remarks to/about you that could be considered button-pushing.
Why do you characterize your response as a mood swing? Even I would call it completely involving reaction--I don't see any mood swings. Are you being treated for your disorder? Do you have mood swings regularly?
This doesn't sound like any situation that a calm discussion can't rectify. Possibly all they know about a bipolar condition is what they've stumbled across in the tabloids. Here's your chance for a little one-on-one honesty. Apologize for being oversensitive, & assure them that you don't hang about with the Hollywood bipolars ... You can't hook them up with anyone. Try to keep a sense of humor if you can.
If you're not seeing a therapist and pdoc, it might be a good idea to try to arrange something.
Roadie