spunk sisters 

When two gay men have received semen from the same man.

James: "Manuel, you too were bred by Jack?"
Manuel: "Yeah, now we're spunk sisters, for real!"

spunk sisters by cumbro February 19, 2019

Spunk Brother / Sister 

A half sibling via your father

ie from his 'spunk donation'

usually applying to promiscuous men with more than one family.

I didn't know you had a brother/sister - Oh he's my "Spunk Brother / Sister"

Underlining the fact that the relationship is not a close one.

Spunk Brother / Sister by Big Superfuzz April 2, 2009