You're (biological or unbiological) sister's boyfriend.
Meghan's unbiological sister is Tori. Tori is dating Patrick. Patrick is Meghan's boyfriend-in-law.
by MeghanMurder May 16, 2010
How you are introduced to people at a party by your girlfriends sister, when the parties in question are over 21 years old and very committed to each other.
Hey Sally, this is my Boyfriend-In-Law,(name), and hopefully soon he will make my sister an honest woman.
by Yankee Fan in Orlando June 15, 2009