Oh boy, it seems like you have compounded problems. Unfortunately the mother in law should have done as described in this article before giving you the device:
What to do before selling or giving away your iPhone, iPad, or iPod touch [or if you received one that had not been erased] - "Use these steps to remove your personal information from a device, even if you don't have it anymore." Click here --> https://support.apple.com/HT201351
In fact if she can still do them now it might clear up the series of hurdles you are facing.
As for the Apple ID Locked message you are seeing --> "If you or someone else enters your password, security questions, or other account information incorrectly too many times, your Apple ID automatically locks to protect your security and you can't sign in to any Apple services."
If the message says your Apple ID is locked or disabled, click here and read what to do --> https://support.apple.com/HT204106
Also note "What to do after [or before] you change your Apple ID [email address] or password". Click here --> https://support.apple.com/HT204071 - "After you change your Apple ID email address or password, you might need to update your sign-in information with any Apple services you're using." - Or if you're seeing persistent disabled messages or repeated requests for the password to your previous Apple ID.
Recover your Apple ID when you can’t reset your password - "If you use two-factor authentication and can’t sign in or reset your password, you can regain access after an account recovery waiting period." - https://support.apple.com/HT204921
If you cannot resolve it yourself try one of these contact methods:
- Click here --> https://support.apple.com/choose-country-region/media-services Select your country, then a product.
- Contact Apple for help with Apple ID account security - https://support.apple.com/HT204169
- Contact Apple for support and service - "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232
I could also post a bunch of links dealing with forgotten security questions but Apple has essentially abandoned security questions and if you don't know the answers then there tends not to be much recourse.
If you see the message "not sufficient information" read this link:
Security and your Apple ID - https://support.apple.com/HT201303
"Without proof of your identity ... and other carefully selected criteria, Apple Support can't help you perform any actions on your account."
Apple does have "Account Recovery when you can’t reset your Apple ID password" ( https://support.apple.com/HT204921 ) which it specifies is available for "when you don’t have enough information to reset your password." However, it also states it is "if you use two-factor authentication" which you may not be if you are still using security questions. You can investigate if this is still available to you (please report back if it is so I know this is a viable option for others in this situation).
In the end it may be easier to get Apple to erase the phone and you set it up with your ID. However, this may require your mother in law still having the receipt for the device and submitting the paperwork: https://al-support.apple.com/#/additional-support
