in my experience there is very little information on the end stages of life specifically regarding brain tumours. http://www.brainhospice.com/
can give you good ideas of what to look out for but don't panic when you see the odd symptom - you really will know when it comes to be 'time'. One thing with brain tumours is that they are generally seen as being a less horrid way to die than some other cancers. I know with lots of stories I've read, and with my father personally, he had very little discomfort because his body began to naturally shut down and he was very out of it. The pain he did have was at the very end, and easily controlled with the use of morphine via syringe driver.
It is difficult but there are all sorts of discussions you are going to have to have while your mum is still coherent. The main things are where to be based when she becomes immobile (my dad chose the living room downstairs because he liked being in the centre of family activity, and having the huge tv), what she would like in the end stages, her will and her funeral. Also, you need to prepare for the idea that your mum may not progress as quickly as you have been told. This for us created an issue as we were having care at home via Continuing Care Fast Track but the PCT became unwilling to fund it further and wanted to move him to a nursing home (understaffed and zero experience of brain tumours, also, he'd have been the youngest there by thirty years!) If your mum is opposed to that, you should have her make it clear to professionals now because further down the line this created conflict with the care coordinator. 'Fortunately' my father became ill in the end stages the day he was supposed to be moved to a home and was able to die in a hospice like he wanted.
Depending on your PCT you may only get a few carer visits a day. A patient with a brain tumour needs more care than this, especially while they are mobile and above all, somebody who is bed bound in my opinion should have personal cares more frequently in order to preserve dignity. You should establish whether she is comfortable with you and your father assisting with these needs, or whether she would prefer professionals. Luckily my father had a critical illness policy so I ended up arranging private carers. For us it was the right thing to know he was always in safe hands, and took so much strain away from myself and my school age brother.
You may be offered night sitters from Marie Curie - they are angels and see themselves as there to support the whole family, as well as the patient. If you get up in the night all the time to check she's ok, this may be a saviour for you. A family caring for somebody all need to make sure they get productive sleep and don't get run down themselves. You can't be strong for your mum if you let yourself get ill.
If your mother has recently been given dexamethasone to control the swelling in the brain, you will probably notice a massive improvement in a few days. However you should also prepare yourself for the effects of long term high dose steroids - namely a sweet tooth and utterly insane appetite. We could not keep the cupboards full for very long at all. Steroid induced diabetes is common but honestly, just give her anything she fancies. Let her make as many choices as possible.
Do you know where in the brain the tumour is? You may be able to predict how your mother will deteriorate and mentally prepare yourself for it, and figure out ways of coping. You will have lots of opportunity to be inventive in order to keep your mother as content as possible. My father liked to fidget and would tell us he was at work measuring up a house for carpets, and he was always using his hands. We bought him a Rubix cube. He did take the stickers off and never complete it... but he sure enjoyed himself.
I can't think of anything else to add, but if you have any questions or need to talk, feel free to PM me.