Short answer: spank him.

Long answer: spank him until he is crying hard.

Sorry that you've got your property ruined by nephew.  My friend had his Wii lens break due to the same thing (his 2 year old daughter stuffed a bunch of coins in it).  Not sure why kids want to stick coins into disc drives (he said she even did it to his cd player in his car, although he was able to fix that one cause it wasn't turned on).  Apparently, the Wii was turned on and it was making terrible noise from the drive spinning with the coins inside. 

I don't let any little kid touch my gaming equipment unless I am present.  I learned the hard way back in GCN era when a cousin deleted my Phantasy Star Online characters (one was about lvl 120 something with tons of rare items).  I could never get myself to try and play that game again after that happened. 

Your options: call Nintendo tech support.  Nintendo phone support is one of the best in any industry.  This is probably not covered under warranty so remove the coins and say the disc drive just stopped working.  If they try and say it isn't covered under warranty after you send it to them then start going off how loyal customer you are and that you are very upset that they can't even cut you a break.  Maybe hang up phone on them then call back and tell them the previous rep didn't help at all.  They then go to code red and start sending offers to you left and right (especially if you follow what I did next to them after I hung up on first rep).  Then send them a message on their website.  I sent them a message after hanging up on them blaming my Wii Sports disc didn't work cause they provided crappy case for it and not a real legit case, and claiming the previous phone rep didn't help me at all.  They were going to have me pay 20 for a new Wii Sports disc but then after the message to their site they called back in 10 mins and offered it to me for free. 

If all else fails and they still won't fix the Wii for free then give your sister/brother the repair bill and make them pay.

Least you got a good thong out of it!

This. 

But if not that ... actually, I don't know the layout of where you live, but I wonder if it's an option to put a lock on the door to the room with the Wii in it, and if your brother objects tell him to train his kid to not destroy people's property and then you won't. 

Or, and this won't help the Wii but at least your games won't get scratched, put all your games in a CD wallet and hide them when you're gone.