This is not legal Advice!
Me prsonally, I think that you have no chance because if it is a shared box.
He will just say that all the contence belongs to his Sister and is just an extra name on the account. Unless you were to get some evidance off of his sister to say the contary.
Then you may be in with a sniff.
You see the problem is in that, he may go to the box but can you prove that.
1> He puts in money and half belongs to you.
2>Or he has been draining your "Joint" account and has put the money in the box.
Tricky