It started when Malcolm was introduced in the hood, as fiancé to Malva Montell. Normally, since Malva and Malcolm were engaged, I would just have moved her in after college. However, Malva wanted to marry a rich sim, so I introduced an unorthodox solution to fulfill that want.
What I did was this: I moved Malva back into her father's house after college, but didn't play the lot. Instead I opened Malcolm's lot and let him host the wedding, thus fulfilling Malva's want to marry a rich sim.
When opening Malcolm's lot, I checked his family tree out, since I know he's a playable sim, even though I've always treated him as a townie. I found that his mother was called Burlin when growing up, and there was my idea that he is related to Kevin. Since Kevin's name is Burlin, his father must have been the brother of Malcom's mother.
Tada! Kevin and Malcolm are cousins!
Admit it, you would never have guessed it! :)