Whatever you do, don't give your husband a copy of the key for the new lock.

Change the fucking locks and send her the bill. Also, explain to your husband that this isn’t okay behavior if he “doesn’t see anything wrong with it” because it’s your turf not hers.

You don't really have a problem with your mother-in-law. Your husband does - if he can't, or won't, explain to his mother, that she is to stay out without an invitation - he is the one you should get rid of. He calls the shots with his mother - you call the shots with your husband.

That will just be a waste of money. Your careless mama’s boy husband will let her copy the new key. You need to change husbands.

Get a digital lock. And maybe talk to your husband about the monster in law

Change the locks

If your husband doesn't see an issue or won't stand his ground with his mother it is time to rethink your marriage. This is likely to be reoccuring and will certainly cause problems later on

Change the locks.

1) if she really did steal the key from your husband, he is not at fault unless he lets her do this. 2) it will be easier just to change the locks then getting the key back. Also, you don’t know if she copies of the copy

then you change the lock and refuse to give him a copy... get a finger print lock and charge her for having to put it in