As my username suggests I also lost my sister and it was sudden. She had a stroke, was recovering then ten days later had a second stroke and died. So I completely understand what you have been going through.
And awful as it sounds I think the suddenness does make it different, not worse but definitely different to someone who has seen their elderly grandparent decline over years.
I was really surprised at the people who stepped up and those who didn't, it was an eye-opener.
That said, I had never been through a close bereavement before and had no idea how hard it would hit me. I suddenly had a new understanding of what friends had been though and even apologised to one good friend that I could have been a better friend to her during her bereavements. As she said, you're now in a club that nobody wants to be in, and that meant I suddenly got it.
Again, with siblings I think people really don't get it. This is someone that all of your childhood was with, your memories of them are so integral to your life. My parents must have got about 40 sympathy cards, I got 4 or 5 which included one from my parents' cleaner!
I had some counselling 4 or 5 months later but it wasn't bereavement counselling, more that there were a lot of things going on in life at that time and I was overwhelmed. It's now 18 months later and at the moment I don't feel I need bereavement counselling but wouldn't shut the door on it entirely.
I'm so sorry for your loss, take care of yourself 