He needs to grant you and/or you sister a power of attorney to deposit his checks (or more broadly, to do banking or handle financial matters for him)--that's one way to do this. He can have one drawn up in his home country and send it to you, but you need to make sure it (and the way it is signed and witnessed) meets the requirements for a power of attorney in your state; a good idea would be to have an attorney draft if for you, though if you don't want to pay a lawyer to do that, you can find instructions, samples, etc. on the internet (just look at several sources and compare, to make sure you have the right and complete information).Or you could mail or UPS the checks to him, he could endorse them on the back and write also on the back, over where he endorses it, "pay to [insert your or your sister's name]" and then send the check back to you; he will have endorsed it for payment to whomever he indicated on the back.