Your best bet is to set a new Lock on the device and a new pin/pw will override the old lock.
Unfortunately, this facility has begun to fail on newer versions of Android on some models, (Google are withdrawing it) but its worth a try here : www.google.com/android/devicemanager.
The thing is you would need the access to her Google account.
If you can find her Gmail address, Google can help if you know some saved memorable information or alternative phone #/email she would have added.
As Madd said, try the carrier first. Bring a photo ID if it's your account, or her photo ID and death certificate if it's her account. They can unlock it for you as long as you can prove you have the right to get into the phone.