Yes, cain is a Scrabble word!
...and is worth 6 points in Scrabble, and 8 points in Words with Friends
cain
Noun
1. (Old Testament) Cain and Abel were the first children of Adam and Eve born after the Fall of Man; Cain killed Abel out of jealousy and was exiled by God
What Scrabble words can I make with the letters in cain?
3 Letter Words
2 Letter Words