As a close relative, you may be able directly recover access to your late Mother’s AppleID account - as you may possess the relevant knowledge. The following information, written from the perspective of the account owner, may be helpful.
If you have forgotten your AppleID or associated password, these can be recovered here:
https://iforgot.apple.com
To recover your credentials, you’ll need access to one of the following:
- Your primary email address mailbox that corresponds with your AppleID
- Any of the secondary/recovery email addresses that should be configured for your AppleID account
- Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID
Unless you AppleID account has not been fully/correctly configured, or has been seriously neglected, recovery of the AppleID and password should not be difficult.
More information about recovery of your AppleID:
https://support.apple.com/en-us/HT201354
More information about recovery of your AppleID password:
https://support.apple.com/en-us/HT201487
When you recover access to your credentials, you would be well advised to log-in to your AppleID account from a web browser - and verify/update any email addresses and trusted telephone numbers:
https://appleid.apple.com
After you have recovered the AppleID and associated password, you’ll be able to resolve any issues with the Activation Lock