kamu is incorrect apparently...
Kamu is one person, kalian is more than one.
Unless there's something weird happening with multiple personalities, you wouldn't normally ask who is thinnest among one person ;)
Why the heck kamu isn't equal to kalian??
Kamu is one person, kalian is more than one. This sentence requires more than one for there to be a superlative "among" them
I believe that in similar context "paling" could be used instead of "ter".
'kamu' = singular 'you' and 'kalian' = plural 'you'.
To use the superlative ('terkurus' in this case), you must have at least three items (people in this case) to compare, so you need the plural.