Every time I say “my brother’s partner” I then have to specify that it’s a woman. Which makes “my sister-in-law” really tempting, no matter how much she’s not the marrying kind.
‘My bitch’ doesn’t have that je ne sais quoi. 
Maybe he doesn’t know that the word “fiancee” actually means formally engaged, he just thinks it means something like “really serious girlfriend who I can see myself marrying one day”?
How bright is the guy? Because maybe he does have a Fiance as well as you and he’s really dim enough to think he can invite you both on a vacation, split his time with you both and get away with it.
Like some wacky sitcom episode.
Didn’t we decide on “jizz jar” in that pit thread about ‘the wife’? Or are we reserving that honor for married couples only?