My sympathies with the loss of your cousin.
Frankly, any Nexus device should be easily restored to factory settings without any add security. I can think of a few things that might be going on. If the phone was under contract from a carrier, it most likely is carrier locked and requires an unlock code to move to a different carrier. For example, if your cousin was a Verizon customer and you put your AT&T sim in the phone, you might get a prompt to unlock it with a code or pin.
It could also be that the account has fallen into arrears due to your cousin's untimely passing or that the carrier was notified of the death and have somehow locked or frozen the account without actually blacklisting the device. If this is the case, there are a few things you may need to do before the device becomes unencumbered by the contract, most of which are up to the carrier's discretion. I have served as executor for a few estates and at the very least you'll need to show them a copy of the death certificate ... or the executor will.
Probate laws will differ by state so you probably will need to check with the next of kin/executor.
If you had access to the device prior to the reset, but afterwards are seeing a prompt for a pin, it could simply be some sort of carrier activation which should be able to be bypassed by clicking through the screen. Did your cousin have the phone rooted and a custom recovery installed? It might be a security feature of the recovery mode (although that would more likely pop up when trying to get into recovery and not after a reset ... just thinking out loud now.)
@KOLIO 's suggestion might be best to simply take it in to a carrier store from where the phone was purchased.
