I [i]think[/i] he would have to be a cousin, since he's plain old Zabrak and she's Dathomirian, which apparently is a hybrid of Zabrak and human?  Things I didn't know.  

Or he could be a half brother, the pure blooded son of her father?  

Either way, I did ask for Diabolic's character not to already have Force powers, so I'd have to ask the same of you.  This might sound like a disadvantage, but other characters will be able to train them, and they'll actually grow stronger faster, at first.  

It's also worth noting that he would not know that the Jedi Order still existed.  They've been in secret for a century or more, and were down to one member for decades at a time.

I'm not sure how I feel about him using Darth Maul's lightsaber.  Though to be fair, I don't know what actually happened to it after Episode I.  I know he reappeared in the show or comics or something.