Mungojerrie and Rumpleteazer Contest!

I hear a lot of debate about Mungojerrie and Rumpleteazer's relationship. I know that they are either one of these three:

Brother and Sister

Mates

Partners In Crime- That's it.

So that's why I made this contest. Here are the rules:

It must be an original story. It has to be about how their relationship formed, and the relationship has to be one of those three! I'm not giving you a limit for the rating, but please, don't make anything TOO dirty.

Thank you! The contest ends September 15.