Sorry you are in this situation OP. It sounds awful. Like all the mumsnet advice, mine is you need to put your foot down now.
Do not let her move in with you under any circumstances - she has two properties. She can sell her mother's house and move into another in the usual way. If her circumstances are complicated they are not your responsibility. If you let her stay at yours 'for a week or so' she will have no incentive to move out.
Tell your DH you refuse to have her living with you becauses she causes you harm. Even the thought of it is causing you harm. Insist he tells his DM she cannot move in under any circumstances. You need the reassurance of him saying categorically no. 'Probably won't move in' won't do - probably suggests it is out of your hands. You can also say no yourself.
If DH would be interested in a wider perspective show him this thread.
You are working hard to support and bring up your family. You should be proud of yourself, as should your DH. Don't suggest you will move out if MIL moves in. It is your home and she is not moving in. She is clearly malevolent rather than life enhancing and you need to protect your safe space.
MIL is about my age. It sounds as though you and DH are fairly young and you have been under her thumb since your teens. Now is the time to put your foot down. Look at all you have achieved and take pride in the fact that you have been stoic and resolute and tried to do the right thing. If you were my dd I would be very proud of you.
Finally, when thinking about what is best for everyone include what is best for yourself. Set aside a part of yourself to stick up for you. Sometimes you have to send out a lifeboat to yourself.
Oh - and if she has a key change the lock.