What is the best diaper for long term sleeping, overnight?

Medline brand extended wear briefs are what I use for my mother at night. I also put large, quilted, washable pads under her to protect the mattress. They launder nicely. We use FitRight briefs for daily use. Both have a softer, less plastic feel. If you go on Amazon and compare all the diapers as far as price, you can see how much each cost per count. The Medline are $1.50 per diaper. The FitRight are 84 cents a diaper.
I’ve been caring for my 86 year old mother for over 2 years. Follow me for tips! 😂

Double diapering doesn't work because of the impervious backing, booster pads are absorbent and allow excess to pass through.

https://www.agingcare.com/products/adult-incontinence-booster-
pads-140480.htm

But I agree with everyone else that 12 is too long, she can always sleep again after you change her.

Wow! 12 hours is a long, long time for any incontinence under garments. For my mom, I use Abri-Flex Premium Level 3 (L3 on the packaging). They are pull ups and I apply a booster pad-- Eterna Dry in the large pad size -- for overnight. They are very good for about 8 or 9 hours and thus far no wet underpads, however, when she gets up there may be a deluge.

A diaper change when she's wet will solve the leaking problem. 12 hours in a wet diaper is a long time

At some point you will have to change her while she is in bed. When I would do this for my Husband there were times when he barely woke during the process.
Or you wake her 6 or 8 hours or so into her sleep to get her to the bathroom to change her.
If you have not already limited her fluids early then you can try that.
There are liners that you can place inside the disposable brief (aka "diaper") but it has to be one that is intended to be used as a liner. Other "pads" have a leakproof backing that will inhibit absorption.