Never let blood ties make a blind judgement call for you. Especially when it comes to your financial well being. Crap like that has costs me 10s of 1000s of $.
If it's a case of
I know he wont pay me back
And he already agreed to then if I were you I'd take some sort of collateral with a time limit on it. Have him sign off his car title or something of that nature with an agreement that you won't pull it out from under him until such and such date. If he scoffs, refuses or pulls the "That's fucked man you know I'm good for it" then just step away and let them handle their own business. If they were "Good for it" then they wouldn't be bumming off from you in the first place.
You'll find that your family will fuck you over faster and worse than anyone else on the planet.
People that don't honor a deal are just as bad as thieves. It's profit through deception at another's cost so it's the same thing at it's basic level.
I've rarely had a family member pay me back and they justify it with "We're family man" and all that crap.
But I've had a coworker I posted bail for pay me back and then some after I told them not to even worry about it in the first place until his court shit was over with (he got caught growing)