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1.
Your mate, spouse, husband, wife, boyfriend, girlfriend, steady and/or lover. Term invented by writer Alden Loveshade that's easier to say and less pretentious than "significant other". Rhymes with "mother" and "brother".
That there's Pat, my new sother.

Pat? What happened to your old sother?

Dude, all Chris wanted to do was cuddles and sharies.
by Alex Firmament February 20, 2009
 
2.
Proper term for a sibling that transitions from male to female.
My brother just said he wants to be a girl. I now call him my sother.
by LonelyBrother March 07, 2012