Are there any helpful suggestions for a relative with mild dementia who experiences severe bed wetting every night?

Depends like stuff does not work for this. These are meant for leaks not full bladder output. Specially when person is in bed.

My mom sleeps with Tena 3XL pads that are 27 inch long and those contain all.
I know as I sometimes use them myself when travelling.

There is no need for any type of plastic sheets etc. as there is no leak ever

I order overnight diapers . Don’t let them on more than 8-10 hours . Limit fluids a bit before bed . I have used the Prevail Air overnights with excellent results . And no UTIs
and I use washable bed pads and water proof mattress cover . But since the change in diapers there’s been no trouble

for fitted sheets, I have been very grateful to find Peelaway sheets. These are so easy to use!!!

Dixcasa: Considerably amend the time of day that she is taking her last drink of liquid. Use a bedside commode as obviously all night in bed isn't effective.

Mu own answer wasn’t suggesting limiting fluid intake, just timing it as well as possible. The greater the fluid intake late in the day and earlier evening, the more like it is to come out in the night hours. Timing it from waking in the morning, it is like to work through the system while M is still awake. It’s certainly true for me. For M, the same amount of fluid becomes easier to deal with. Perhaps give it a try?

My mother (96) will occasionally sleep through the night (yay) and when she wakes up, her diaper and pad probably weight 2 pounds.

Underneath her sheet, on top of the mattress cover, I have a disposable bed pad. On top of the sheet, I have a 36"x36" waterproof cloth bed pad (I have two of them so I can switch them out when one needs washing).

https://a.co/d/0V83J08

I don't limit my mother's water intake during the day because the hydration has kept her from having a UTI.

On her, I have Always Discreet underwear.

https://a.co/d/eBRJQJh

And inside the underwear I have Tena overnight pads. I've tried various brands but the Tena are super soft and don't irritate around the edges.

https://a.co/d/a9VDHKa

Rarely does she wet outside of the underwear, but if she does, it's minimal and I just change her clothes and wash the bed pad.

There are three things that have made a difference in my family, With me, the Innovo system worked, which used a 'TENS' machine-type of approach, to strengthen the muscles around the urethra (electrodes in spandex bicycle shorts and you flip a switch and it contracts the muscles for half an hour.) After three weeks, I didn't need it any more, as leaking was over. With my mother, not drinking after 7 pm helped. With my sister, who was wetting the bed multiple times a night, we tried a keto diet (or at the very least, getting rid of sugar and corn syrup-laden desserts and snacks--sugar can make you pee!) Within two weeks, no more bed-wetting, which allowed her to sleep all the way through the night, which helped not only her dementia, but her attitude! She wasn't eating a lot of sugar, but did have a cookie in the afternoon and another sweet dessert after dinner. That was enough to cause the problem, apparently, and now she's feeling a lot better!

The PCP should be informed as they may need to further assess the potential causes of the bed wetting and, then proceed with best options to consider. Obviously some form of adult diaper and bed pads can be purchased and tried but even with these there may be one that is better for your particular loved one. There are also some other assistant devices that medical professionals may recommend or talk about. Having the patient assessed by PCP is a priority as there could be any number of reasons contributing to the bed wetting.

While it might not be a way to manage this, once the waterproof sheets and mattress protector are on the bed, add more blankets on top of top and see if she can wear less or no clothes during the night ... to avoid needing to wash wet clothes the following day.

Be sure to use barrier cream(s):

https://www.amazon.com/s?k=barrier+cream+for+incontinence&crid=3GDZSLE1IDJ6U&sprefix=barrier+cream%2Caps%2C199&ref=nb_sb_ss_ts-doa-p_2_13

These might be good as they appear bulky and could accommodate depends, or perhaps doubling up on depends and they say they aren't noisy:
https://www.amazon.com/Leakproof-Underwear-Incontinence-Washable-Waterproof/dp/B0BQJ43BY9/ref=sr_1_16?crid=1CKVTXHRAU51C&keywords=adult%2Bincontinence%2Bclothes&qid=1693498491&sprefix=clothes%2Bfor%2Bincontinence%2Caps%2C162&sr=8-16&th=1

Try googling "clothes for incontinence' - I started there. There may be more than what I could find in a short time.

Gena / Touch Matters

Certainly have a plastic sheet / protective covering on top of the mattress.

I found a waterproof top sheet:

https://www.amazon.com/Waterproof-Sheet-Flat-Sheets-Queen/dp/B098MCS16M/ref=sr_1_1_sspa?hvadid=580694638051&hvdev=c&hvlocphy=9032096&hvnetw=g&hvqmt=e&hvrand=15636583947006154776&hvtargid=kwd-11238269676&hydadcr=21878_13323245&keywords=waterproof%2Bflat%2Bsheet&qid=1693498142&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

And you likely can find more options (this one happens to be queen size but I saw more advertised. Google "Waterproof flat top sheets" .
You could double up on the depends and/or find ones that are for heavier flow.

Gena / Touch Matters

There are home administered sleep apnea test available now. Contact a Pulmanary doctor, ENT or other specialist who helps with sleep apnea in your area and ask if they can order for your relative. Many who have night time wetting (nocturnal enuresis) have sleep apnea. If she does it might even improve her “mild dementia”.

This may not even be equal to the problem you're facing. However, I had the same problem when my 87 year old wife first become afflicted with dementia. At the time I had no idea of the use of, or difference in females pads. When I learned that the ones my wife was using were noted as number 2 - I learned that there were pads that went as high as number 6 - some specifically for incontinence. From going to changing bedclothing almost daily. when she used the number 2 - to changing and washing only once a week. using the number 6 Incontinence pads. My wife refuses to use the depends type pull on panties, only the pads.

We had the same issue with my mom. Took her to a specialist and found out she has an overactive bladder. The doctor implanted a ring (not sure of the correct name) and it’s changed ever 3 months, plus medication. She still wears depends at night and her bed is padded, but the bed is dry every morning. During the day she wears incontinence underwear (Amazon) and thick period pads. If the pads get wet she pulls it out and puts in a clean one.

The liquid obviously comes from drinking. Try reducing fluids in after lunch, none from evening onwards, then lots in the morning. It may reduce fluids out at night, and make the problem more manageable. That’s my personal experience, but others may have comments.

Of course getting her to bathroom would be awesome but reality from someone over the year mark with bedridden husband, what can you do all the time for long haul needs to be realistic for your health and ability to sleep. I tried boosters, underpads ect and pretty much everything leaks, especially if you're on a budget for supplies. If you got access to a good washer the snap style cloth diapers are a good option. Amazon has a 5 pack with inserts thats a good trial to see if it works. If you need more adsorption power regular flat baby diapers are a soft booster you can fold where needed and for comfort. We also got washable underpads now and I don't have to worry about running out or throwing out what money we have.

Thank you. I’m in same situation with my 84 year old Mother. 😞

You may have to get her up a few times a night so she can use the bathroom or bedside commode. You should also being adding extra pads in her Depends for overnight, and putting down extra chux pads under her as well.
And then of course you can always have her urologists recommend a catheter for her. I believe the Purwick catheter works well, but insurance doesn't cover it and it's kind of expensive, but obviously worth it if it saves you time and trouble and lets you sleep.
Incontinence is often the straw that will break the camels back and lead someone to have to place their loved one in the appropriate facility. You may be there and it's ok if you are.
I wish you well in figuring out what the best solutions are for your situation.

I did a site search for you - this is a question that come up often

https://www.agingcare.com/search?term=best+incontinence+products