Password

You can also add 2 Factor Authentication to your account for more security. If the attempts continue, you might want to consider a name change.

Link to contact Apple Support in the article.

Apple ID has been compromised

Note the restrictions in the article.

Apple ID Name Change

 What to do after you change your Apple ID or password - Apple Support     I suggest doing this before you change your Apple ID or password.

 Apple ID -Two-factor authentication

 Two Factor Authentification availability

 Apple ID - Using app-specific passwords

Apple ID - Two Step Verification App Specific Passwords

Apple ID - Two Step Verifications FAQ

First, do you mean password or passcode?

Change your passcode and don't tell them the new one. Of course they can't use your device if you do that. I don't know if Screen Time offers anything along these lines. A passcode is a pretty fundamental level tool to stop people from doing unwanted things on your device.

Use Screen Time on your iPhone, iPad, or iPod touch - https://support.apple.com/HT208982

Perhaps back up and tell us what exactly is happening to make you reset your password. Is it really being changed?

Frankly the word "hacked" os over-used. Is your wife one of those wonder, female hackers I see on TV who can turn to a computer and then 5 seconds later say, "Okay, I'm into the NSA database!?" Or is she "hacking" by coming up to you and saying, "Sweeties, I can never remember the name of your first pet, what was it again?"

I strongly suspect what you are seeing isn't "hacking" but you misunderstanding some other feature or message.

If you mean password, are you sharing an Apple ID with the family member. If you are, you should each have your own Apple ID. Apple IDs aren't meant to be shared.