how to unlock an iPad?

I'm very sorry about your loss. The above article won't likely help you unless you know your mom's Apple ID and Apple ID Password, which you likely don't.

Instead for family members, Apple has a specific process you can follow to get their help in unlocking the device. See this link and this is what you're going to need to do --> How to request access to a deceased family member's Apple account - Apple Support

You need to take it and the other documentation outlined in the support article we provided along with the iPad to your Apple Store Genius Bar. That would include a copy of the death certificate and something from the estate granting you ownership rights.

Hello

Apple has a perfect article for your situation. If you forgot your iPad passcode - Apple Support

Yes, it really helps if you can provide the original purchase receipt. Good luck with this. The process link I provided is really the only way you're going to be able to use the iPad.