Oh my god, that is so shocking... I really feel for you brother.
Like Idol said, you obviously can't blame yourself, though you are grieving so you might think of illogical scenarios that will multiply any (unjustified) guilt. A few years ago a distant uncle of mine died of a brain tumor, and despite it being totally illogical, I sometimes blamed myself for something which had no relation with his death. To make a long story short he had a daughter who liked me, and he liked me so much, he really really wanted me to marry her. This made me repulsed by her, and he was really pushing her on me (not literally of course). Basically I had a nightmare one time that he was sick in bed and using that to make me marry her... a week later we found out he was ill and dying, which was nuts. Despite none of it making any sense, a part of me felt guilty... which is obviously ridiculous.
If anyone is responsible, then it is your grandmother's daughters and son who neglected her and had a duty to be closer to her and thus realize what state she was in. But if they actually knew what was going to happen, they wouldn't have neglected her, so there is obviously no point in blaming anyone... least of all yourself obviously.
I am sure a part of you knows you aren't to blame.
It is uncanny how so much of this reminds me of my family. My maternal grandmother is loved by all her nine children, but almost all live in other countries and can't give her time. Her sons haven't seen her in so long, it is ridiculous. Some haven't seen her in years, some haven't even spoken to her in longer. Part of it has to do with distance, and part because their wives are the crazy fucking type.
Every time I see my grandmother she just holds my hand and starts talking about her kids. Then her eyes water up and she cries. I've written to my uncles on many an occasion, but they always make empty promises. It isn't that they love her, it is just... I don't fucking know what it is.
Anyway man I hope you get through this well my friend, and I hope your aunt and uncle gain some sense.
I just want to relate a verse from the Quran. In Islam there are few sins greater than neglecting your parents.
"...And your Lord has commanded that you worship none but Him and that you be kind to parents. If either one or both of them reach old age with you, say not to them a word of disrespect nor chide them, but address them in terms of honour. And lower to them your wing of humility and say, "My Lord and Sustainer, have mercy on them both as they nurtured, cherished and sustained me in childhood."
Here is another interesting one:
'A man came to Prophet Mohammad (PBUH) and asked permission to go to Jihad (struggling for the sake of Islam). The Prophet asked him, "are your parents alive?" The man said, "yes". The Prophet responded, "then strive to serve them."
Anyway I guess it is the same in all cultures, religions etc. Respect and serve your parents damnit!!!!