I have a Christmas cactus that is over 60 years old, probably older than that. It has slowly been dying, the original plant has died now, only plants that were started from it are still alive, but they are dying as well. We have tried repotting. It is in an area that is well lit but without direct sunlight. It gets watered as needed.
I love stories of Heirloom plants!
The plant does appear old, quite woody at the base and in general, not to healthy.
The pot seems very large for what remains of the plant.
The over sized pot can lead to over watering and root rot.
In this case I would recommend saving the plant by propagating the ends that appear more healthy.
Follow the directions in the link below and start the cuttings in small pots.