My mom removes her overnight underwear in her sleep and then wets the bed. Any suggestions for getting her to keep them on or put on a new?

Here is a link to an Alzheimer's anti-strip pajama that will prevent your mom from removing her overnight underwear:

https://www.amazon.com/Talit-Inc-Womens-Alzheimers-Clothing/dp/B07NRSY32X/ref=sr_1_1?dchild=1&keywords=alzheimers+anti+strip+pajamas&qid=1595526026&sr=8-1

Overnight Depends are thicker than the daytime briefs, but if your mother is removing them at night, it's a moot point which one is better to use. If she's wetting the bed and you find yourself changing sheets continuously even WITH an anti strip pj, try these, they are FANTASTIC:

https://www.amazon.com/Medline-Extrasorbs-Drypad-Underpads-Permeable/dp/B07GYZNDY8/ref=sr_1_8?crid=1U57RJGC6G6FF&dchild=1&keywords=chuck+pads+disposable+for+adults&qid=1595526169&sprefix=chuck+pad%2Caps%2C187&sr=8-8

Good luck!

You can try a "Onesie" that will prevent her from removing the brief, either a tab type or pull up type.
If it is a tab type you could try putting it on backwards so that the tape is attached at the back so it is more difficult for her to remove.
If she is wearing 2 piece PJ's you could try attaching the top to the bottom with a snap or a hook and loop closure (Velcro is one type)
Get a good absorbent underpad that you can wash. They seem to absorb more than the disposable ones.
If she does not have a hospital bed or hospital type mattress you might consider that. They are washable and do not absorb like a regular mattress. (I can not tell you the number of times that with an absorbent pad, and an absorbent tab brief I had to mop the floor because my Husband would not just "wet" the bed but there would be a PUDDLE under the bed.!)
There are liners you can use just make sure the liner you get is intended to be used with a brief some have a "waterproof" lining on the back, those are intended to be used with underwear not an incontinent product.

Similar question with answers at this post:

https://www.agingcare.com/questions/what-can-be-done-so-my-mil-would-stop-taking-her-diapers-off-in-the-middle-of-the-night-460371.htm?orderby=recent