Oh Gilbo, I am so very sorry to read your post, you must all be utterly devastated. Of course you are sad and angry and confused, who wouldn't be, and I hope you are finding the spaces you need to let those emotions out.
There is no one 'right' way to face this awful situation, but however you approach it is right for you. Try to take things a day at a time. If you are able to spend time with your Mum, try to focus on happy things, memories that make you smile and laugh. If she has favourite music perhaps you could play that. Or read to her from favourite books. Look at photos and talk about them. Perhaps you could massage her skin with her favourite lotion. If she has 'better' days and the weather is suitable, perhaps she can be taken out to places she loves. There is still time to make some new, precious memories, to take photos of her with her grandchildren.
I firmly believe that our loved ones do not leave us until they know we are ready to cope without them. Of course it will be very difficult, of course you will miss her depserately, but you will know that she knew you could do it.
Make sure that you have time to enjoy your son, and time for yourself, the best you can. If you exhaust yourself rushing around, you will be no use to anyone. Do you have a close friend you can confide in?
I wish I could make it right for you, but I can't. I do know you will find suport on here fromforum posters, and that if you phone the helpline people 0808 800 6000 when they are open again, they will offer you a listening ear and practical help.
A cyber hug from me to you.