First of all, cudos to you for taking care of your mother and keeping her home where she should be!
I have a quadraplegic 19 year old daughter, and I also take care of my disabled ex-mother in law. The least expensive solution for this problem I have found was buying the baby bed mattress protectors at the thrift store, cutting the sides off them, and putting them sideways across their beds, putting a folded flat sheet over that for a draw sheet.
They are waterproof,they keep the rest of the bedding dry, and they don't slip. My daughter sometimes has to be changed 2 times a night, so I do this twice on her bed. Remove the top(wet) one, and she is on a fresh one, all in one roll. She does move around in the bed, but it's rare she gets off the mattress padding.
You can also get "diaper doublers" at the medical supply company. They are lifesavers!
I have also seen back sleeper women use the male depends backwards for nighttime.
you are gettign wonderful suggestions on here, and I will watch this thread. Even after 19 years of 24/7, I am still open to suggestions! God Bless.