Stepbrother-In-Law

The title given to someones brother of sisters boyfriend if they're not married

Juan: Who's this Harry geezer?
Tom: Oh he's my sisters boyfriend, that makes him my stepbrother-in-law
Juan: Surely he's your stepbrother?
Tom: Nah fam, he's not married to my sister

by why_am_1_doing_this May 01, 2019