Although I wish I could say that Clint reads this board, all we know is that he's aware that it exists. He's never posted here, that we know of, and this wouldn't be the best place to hope he sees your message. We're just a fan board where fans discuss Clint's movies. A few of the members of this board have been lucky enough to meet Clint, almost all of these circumstances have been by visiting Carmel and spending time at the Hogsbreath Inn. The chances of an actual run-in are very slim, though.
You could also try writing Clint at his official mailing address, which is:
C/O Malpaso Productions, Warner Bros.
4000 Warner Blvd,
Burbank CA, 91522
It's a nice thing you're doing for your brother in law, and I hope you have success.
Let your brother in law know the Clint Fans over here wish him all the best in his fight.