It sounds like your mother would be well served with a couple of implants to support her dentures. Typically, the lower denture is the denture that gives people the most trouble. There is very little tissue area contact to support the denture. As little as two implants can give the denture phenomenal retention. Quite literally, the denture goes from slipping all over the place to being rock solid. Not only is it secure but sore spots are eliminated as well since the support goes to the implants instead of the tissue.
The upper denture covers the palate which gives it suction. A well fitted upper denture can give most patients support they need. If the upper denture is very old it should be replaced. If it is less than ten years old, it can be relined with new acrylic for a just like new fit.