Mdpatrick25,
I have talked with Nick at Yankee Mattress and he is knowledgeable and experienced and I think highly of him and the quality/value of the mattresses they make. You can see some comments about them here and he would certainly be well worth a visit. (NOTE ADDED: They are also now a member of this site)
[quote]I’m thinking a firm latex mattress with a soft top may work?
I would consider memory foam but they seem pretty expensive and I’m not sure how they hold up.[/quote]
Each person can have their own preferences as far as the type of material they prefer to sleep on. Latex is also a more costly material than memory foam. Regardless of the material that your testing indicates that you prefer … it’s important to make sure that it’s a good quality version or grade of that material because all materials have lower quality and less durable versions and higher quality and more durable versions.
I would at least read the tutorial post here which has all the basic information, steps, and guidelines you will need to help you make the best possible choice.
You and your sisters seem to be scattered all over the country but they are certainly welcome here when they decide it’s time to buy a mattress 
I’m looking forward to your feedback if you decide to visit Nick at Yankee Mattress.
Phoenix