if it looks like a blister it could be a friction blister, often seen in the elderly, unlikely to be a pressure sore as she is able to get up and move, just getting up and moving will usually prevent pressure sores. When applying any cream to a suspected pressure sore, be very careful not to massage it, but to just rub the cream very lightly on to the skin, massaging a pressure sore can do a lot of damage. You should immediately inform a GP if you suspect a pressure sore, remember that the damage is done on the inide, if you are seeing damage on the outside, it is far worse on the inside!