Dementia MIL keeps taking her diaper off. Any suggestions?

When my kids did that (pulling the Velcro tabs off their diapers) I put them on backward. They never figured that out, but your mom might. Still, it's worth a try.

My Dad also pulled off his diapers every day, sometimes it seemed he even did it in his sleep. He also wadded up the disposable bed liners and pushed them out of bed. All his life he was sensitive to inseams, tags, rough fabrics and things that were binding so it was no surprise he found diapers unbearable! His dementia was advanced enough that he didn't seem cognizant of the fact he was wearing diapers so it wasn't like he hated them for what they represented (physical failing and dependency).
Check for a UTI first. Then check for proper fit, as mentioned before.
After Dad toileted, and his aide gave him a bath, we would let him lay in bed for a while naked, with a cloth chuck underneath. He seemed so much more comfortable, and he was all tucked into soft sheets and a warm blanket. He couldn't stay too long, or would soil the bed, but an hour was usually safe. It gave his skin a chance to dry completely.
We also finally noted that he pulled off his diapers when they were not completely dry. Even a tiny amount of urine was uncomfortable enough to make him remove them. We learned to use a liner that could be removed when there was a bit of leakage instead of changing the diaper for every few drops of urine.
When people are eating and drinking so little it's amazing how much urine is still produced! It's very concentrated and therefore more irritating to skin than normal, dilute urine is.
Hope this helps. Best of luck to you.

Onsies would help
Putting Pj's on that have snaps or Hook and loop closure/Velcro that you put on the bottom of the top of the pj and the waist of the bottom.
Putting a Tab Type brief on backwards makes it a bit more difficult to get off.
Make sure the Tab type or pull up briefs you are getting fit her properly. If they are large they can bunch and feel uncomfortable (like they are comfortable any way..NOT) If they are small they can bind and hurt.
There are cloth underwear that you can place liners in that would work as well. You can get them in different absorbency.
I did chuckle at ZippyZee suggestion of Duct Tape. It would work but it is difficult to tear off a strip and put where it needs to go if you are the only one doing a change and the adhesive can be irritating if you get it on the skin. Add to that skin on older people is thin and fragile and tears easily. So if you use it take care not to get tape on the skin.

Duct Tape.

Lots of it.

Perhaps time to put mother in law into a facility?

You asked the same question earlier this month: https://www.agingcare.com/questions/how-to-keep-mil-from-removing-diapers-461036.htm?orderby=recent&page=1�

I am curious about something. You and your H recently removed MIL from memory care because they didn't watch her closely enough and she fell a lot, yes?

You seem to be the one who is on 24/7/365 duty with her. You were the one who endured sleepless nights in the beginning. Why is that? Why isn't your H involved, as it is HIS mother?

NorthShore makes an adult onesie that may or may not help, but it also may be worth a try.

There are also one piece pajamas available online, as well as garments that have soft closures at the back that may help her limit her access to tearing or pulling at her under garments.

Are you using the large, pre-moistened bath sized cleansing towels that don’t require water, for cleaning her? Although they don’t solve the problem, they may make her care less wearying to you.

Thinking of you, and hoping that you are getting essential rest and time to yourself.

The make specialty clothes that fasten in the back and are harder to remove, check companies like Silverts, Buck and Buck or Amazon for Alzheimer's clothing.

Make sure she doesn't have a yeast or bladder infection that is making her uncomfortable. She may be trying to get to her "private parts" to scratch or otherwise comfort herself. She may have "focused" on destroying whatever is around her or she likes to tear things. Does she also pull on her clothes that don't come off as easily as a disposable diaper? Give her something else to destroy (paper for example) or to keep her hands busy (folding washcloths, a doll that she might be able to undress) Best of luck with this challenging behavior.