Hi - I'm so sorry to read about your mum. My lovely dad was diagnosed last august with a GBM and as you are feeling now, it totally shocked us all at the start of this journey. I couldn't believe that my fit, healthy dad and grandad to my 3 kids was diagnosed with a brain tumour and then to digest the poor prognosis news.  But I will say that you do quickly adjust to you 'new normal'- that's not to say it's easy, but what else can you do? I've learnt to take it a day at a time and make sure I have as much knowledge on the tumour, the treatments and ask lots of Qs at every appointment. You will feel a bit more in control once you get to grips with understanding the cancer and building relationships with the Doctors's and other health professionals you'll meet along the way. 
I've been overwhelmed with how lovely the doctors have been and even with a grim prognosis there are quite a lot of positive stories thay give you hope of time and quaility of life. We've had some touch and go moments but modern medicine is amazing and given us more time with my Dad. 
Like your mum, my dad suffered set backs which meant he had some prolonged hospital stays which is an anxious time.  Try be there as much as you can for her and say the things you want to say. Don't forgot to take some time for you though so you can digest what's going on and spend time with your little family. Maybe draw up a rota so between family and friends they can visit when you take some time to breathe. 
It really is an agonising time and I wish I could say more to help but as I said earlier, after the initial shock I've learnt to enjoy each day I have with my dad and not look too far ahead as my head can't cope with that.

My kids know grandad isn't well but they really cheer him up and when he was in hospital he loved hearing their voice notes and videos so keep sharing those with her. 
Finally, try talk to friends and family about how you feel - saying it out loud can help and it's ok to be upset, angry, scared etc. sending you a big hug xx