Spunk brothers 

When 2 good friends both ejaculate inside the same woman and the sperm joins together into 1. Similar to becoming blood brothers through pricking each others fingers.

John and Paul where so happy that Emily let them use her to become spunk brothers

Spunk brothers by DonBay March 7, 2020

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