What is a good mattress pad to prevent sores?

Hop....very kind of you. My Mom has been on hospice since early May....though she has rallied and is "hanging in there" at this point. BUT....since she is also mostly bedbound, the very first thing hospice did was send a special mattress....it's air-pocketed.
If I were you, I would contact her hospice provider to inquire about this since it sounds like you are not in her immediate vicinity.....it may, as in my own Mom's case already have been addressed.....wishing both you and your Aunt the best ...

Hopalong, so sorry to read about your Aunt, and how sweet you want to send her things to give her comfort.

If your Aunt is on Hospice, they usually order a hospital bed that has a gel mattress. You might want to check to see if that is part of the Hospice in your Aunt's location. Chances are your Aunt is on meds to help make her more comfortable.

Send her warm fuzzy socks... lotion with lavender.... pretty greeting cards saying "thinking of you".

the best (in my opinion) is the stuff they call "egg crate" foam or convoluted foam. If you can, also send a soft fleece throw for topping her blanket. Very comforting for her skin.