"Whose is the turtle?"
I would say the correct English translation should be 'whose turtle is it?', no?
Apart from that, I'm guess the literal translation of this 'kaplumbağa kimin?' is 'turtle whose?', or more... 'turtle whose is?', right?
And constructing 'kimin' as a genitive... 'kim'in'... like... Baris'in... only you take out the apostrophe because 'kim' is not a name.
Or maybe I got all wrong.