KISSING COUSINS
(not original title)
From The Enigma 
NPL Magazine - June, 2014
NEXT LINGO (adapted)

[The two answers to this flat differ by a single letter, which is found at the end of the longer word.]

My second cousin once removed
Kissed me on the lips.
Some said it was a sin, but she said
"What's their problem? You're not
My brother, just _ _ _ _ of _ _ _."

Answers:

KIND
KIN