I understand that you are having a really hard time now, with lots of bad news and difficult changes happening just within the last 3 months. Your main worry seems to be about the severity of your mum's cancer and particularly that she keeps saying that she will be dead within 2 weeks.
There can be many reasons why your mum keeps saying this. When people have a serious illness like a cancer that has spread, there is of course a real worry that they may die of that disease. People sometimes then ask the doctors how long they can live and mostly it is very hard for anyone to estimate that. When the future is very uncertain and also depends on how the treatment is working, doctors can sometimes say things like "You can live anywhere from 2 weeks to 2 months - or maybe even 2 years". I know that's terribly vague, but it shows that often doctors are very uncertain themselves about how things will turn out in the future or how long someone can live with cancer.
I don't know why your mum talks about 2 weeks. It can be that she thinks that's what the doctors have told her. It can also be she just thinks, as many others do, that if her cancer has spread, then she will die very soon. It might be she knew someone who died very quickly with cancer. Some people say things like this if they are really scared of dying and they need more support from other people. Of course, in some situations, people say this because it may actually be true, but it seems that in your situation, your think it is not. The fact that she is having radiotherapy treatment also suggests that the doctors do not think she will die very soon.
I am sure your mum is very worried about you as you are now staying with your dad, whom you say is an alchoholic. This must be very hard for both you and your mum. It may be that she is trying to prepare you for the fact that she might die in the future and that maybe then you will have to go and stay with your dad. As I don't know how your dad's alchololism affects your life, I can not make any comments as to whether this would be the right thing for you. I would imagine that you and your mum would talk about this and get help from professionals in dealing with such a situation.
Perhaps the best thing for you to do now, is first of all to tell your mum how worrying it is for you, to hear her talk like this. Tell her that it is better for you to know the truth, as that can't possibly be worse than what she is saying about dying within 2 weeeks. As you are now staying with your dad and your mum is having treatment and is probably quite tired and upset herself, I would also recommend you to talk to another adult you trust about your situation. This could be someone else in the family, a family friend, a teacher or similar. There will also be counsellors and social workers availabe through the hospital where your mum is having her treatment, and it may be a good idea for you and your mum to speak to someone there about all your worries.
In addition to this, I recommend you to have a look in the various sections on the riprap website. There are many resources in the support section with information on 'support in your area' and 'third party links' to other national organisations that may be relevant for you. If you would like to get in touch with others in the same situation as yourself, I recommend that you have a look in the forum and either reply to someone or post your own message here.
I am sure your mum only wants the best for you, which is also why she wants to get you a kitten to keep you company. Talk to her and please get back to us again if you need any more advice.