Apple cannot and will not "unlock" - they will not give you access to someone else's information. They can help you erase the iPad (which removes the passcode lock) so that you can use it "clean." To erase, see https://support.apple.com/HT204306
If you just want the iPad unlocked so you can use it for your own purposes - the question is whether the iPad is single-locked (just a passcode lock) or if it's double-locked (passcode lock plus Find My iPhone Activation Lock). Here's more about Activation Lock: https://support.apple.com/HT201365
If you also want the information on the iPad - no way to get that, unless your relative kept a backup in iCloud or iTunes that you can access (possessing Apple ID password for the iCloud account). If there's a backup, you can erase the iPad to remove the passcode lock, then restore from the backup.
Reach out to Apple Support by phone. Make sure to have the death certificate available. You may have to send it to them. Provide them information so they can link the death certificate to the iPad Mini and ask for their help in wiping the system so it can be re-used. Be nice and you may get lucky.
If you're asked for the passcode, then you haven't put the thing in Recovery Mode
Connect to computer
Hold power button and home button until you see the "connect to iTunes" screen (iTunes logo and Lightning cable).
On computer screen, you'll be given option to Update or Restore iPad. Choose "Restore"