My wife gets 12-13 hours of sleep at night and is now starting to sleep during the day. Is this normal with dementia?

My mom sleeps 12 - 14 hours a day and sometimes still takes a nap after dinner.

Normal.
It is also one of the signs of decline that hospice used to keep my Husband on Hospice when it came to re-certify him.
I think towards the end my Husband was sleeping 22 to 23 hours out of the 24.

Yes, as her disease progresses she will sleep more and more.