You are asking very serious and very important questions. Is there anything to do to help it and is there a limit to how long she can live?
It seems from another message you wrote to us that what your mum has got is in fact a breast cancer that has spread to her bones in the back. This is often called secondary breast cancer and is not the same as bone cancer that has started in the bones.
I understand from your other message that you have been told that "this is a cancer they can't do anything for". When a breast cancer has spread to another part of the body - like the bones in the back - it ususally means that there is no treatment that can completely take it away or cure the illness. This is very hard to accept, but it is important to remember that often though, the cancer in the bones can be treated so that the cancer may stop growing for a while or grow slower. They can use different treatments for this, usually radiotherapy, chemotherapy or hormone therapy. The doctors who are treating your mum will know if your mum could make use of such treatment and which of them would be best for her.
As I said, this kind of treatment can not cure the illness and it is very hard to say how long someone can live with a breast cancer that has spread in this way. Some people can live for many years, but it depends very much on each situation and things like how far had the cancer grown and how well the treatment works for that person. It is impossible to say without knowing that person. Your mum's doctor will know her situation in this medical way and he or she will know more about what is likely to happen with her.
I realise that you will have a very hard time now over Christmas and I hope that you will have lots of support around you. Don't be afraid to talk to others about how you feel, as it is important not to bottle things up inside you. You could also check out the forum on the riprap website and either reply to any of the other teenagers out there or write your own message in the forum and see if anyone out there feels the same way as you do.
Please also get back to us if you have any other questions of if you need any other advice.
All good wishes from riprap