I think it is wonderful that you are willing to help with the house. From your email, it seems like the house is more than just messy. I have experienced, first hand, the issues that go into cleaning a house where the home is close to condemnable. The main issue is what to get rid of, what to keep, and what to donate. Unfortunately, the home I helped clean did not have anything worth salvaging. Put all papers into folders and go through later.
My suggestion is to make sure when you mention the "Cleaning" to the sick mother, you give her the peace of mind that you will not get rid of anything you think might be of value to her. Have her write down specific items that mean something to her. Hoarders tend to be obsessive about items that you and I might deem useless. Buy a TON of plastic containers, get lots of help from those that are not judgmental, and go at it. I would not have the Mom involved in the actual clean because she will not get rid of anything. Unfortunately, you will (most likely) not change her behavior, but you will be able to give her some order in a life that is now very stressful. Good luck and God bless!