A would-be brother-in-law who is in a commited relationship with your
sister but isn'
t legally married to her.
These relatives are awkward to introduce because they're not necessarily your friends, but technically not your relatives either. You don'
t want to insult him by going into detail about his relationship with your
sister in public, but if you call him your brother-in-law, awkwardness might arise when people glance at the non-existent wedding ring.
This also applies to would-be parents and sisters-in-law.
Jack: Hey Ted, why don'
t you invite your brother-in-theory onto our softball team?
Ted: Are you kidding? Then my
father-in-theory's gonna want to join too. There's a reason I didn'
t marry into that crazy family, ya know.