She needs psychiatric or psychological help as hoarding is a psychological condition similar to Obsessive Compulsive Disorder. I don't mean that in a horrible way. She will have an emotional attachment to everything in her house, even the stuff we perceive as being rubbish. There are people who can help her, but it's something that needs to be a gradual process. She will give a (valid to her) reason for hanging on to everything she has. She will have a 'use' for everything, whether it be empty soup cans or newspapers. 

It's not just a matter of going in and cleaning the place up as she will just start collecting again and be back to square one in no time, as you've already seen. She needs to deal with the root cause. A good place to start is with her local doctor, who can then refer her onto the appropriate specialist. There are also professional cleaners who deal with hoarding clean ups. I wouldn't just allow them free range as it's quite possible she is hoarding things of value. 

When the clean up starts, it's a good idea to have her psych there with her, so her emotional state can be monitored, as she will be on an emotional roller coaster. It could take months, but she needs to be able to detach herself from all her stuff, which the psych can help her with.

Good luck with it all. With the right help, she will be OK.