Oh my love, I am so, so sorry :(
I am so very sorry for your brother too, that must have been such a shock for him and having to see her like that will stay with him, poor kid. In time try to make sure he gets some counselling x
Please don't feel bad that no-one had seen her since Thursday, it's really not a long time.
I don't know how to say this really, but, your relatives don't sound very nice. No-one nice says 'it's was to be expected' - not even if the person was dying with a terminal illness, let alone when not, it's just not what you say is it?! I'd give them a wide berth if I was you and talk to your MIL & friends, they will want to help you and sadly, plenty of them will have had experience too.
As others have said, you need to speak to the coroners office - they will help you. Then you will need to choose a funeral director and they will help you too.
I am sure your Mum knows how much you love her - they just do! Just as you know how much your DS loves you. I am sorry for his loss too, I know how sad it is when you know that they wont remember, but if you keep talking to him about her and about how much she loved & adored him and keep her photos around the house, he will know he was special to her and that his Gran loved him - it does make a difference.
I really am sorry, 57 is no age :(
x