What does Brother-in-theory mean?

Brother-in-theory meaning in Urban Dictionary

A would-be brother-in-law who's in a commited commitment along with your sister but isn't legally married to her.These family relations are uncomfortable to introduce because they're certainly not your friends, but theoretically perhaps not your family relations often. That you do not wish insult him by starting information about their commitment together with your cousin in public areas, however if you call him your brother-in-law, awkwardness might arise when anyone glance at the non-existent wedding ceremony ring.This also relates to potential parents and sisters-in-law.