So sorry for you OP.
I lost my sister when I was 15, she was 24. Extremely rare immunity disorder, literally one in a million chance. She was poorly for a lot longer that your DB, but my parents shielded me from a lot of what was going on so it was a massive shock when I found out she'd been on life support and they had to switch it off.
Life will never be the same again, but it will be ok - I can promise you that.
At the moment, you don't need to worry about what you're supposed to do. At the moment, you can cry, be angry, be sad, be happy when you remember great times you had together. Just feel what you want to feel.
When my sister died I listened to a lot of Coldplay, mainly 'a rush of blood to the head'. There is a line in 'the scientist' that goes 'nobody said it was easy, nobody said it would be this hard'. It will be hard, so don't feel bad for not being ok.
I am now closer than ever to my brother and other sister and we've all been blessed with children of our own. We will always miss our sister but our lives are happy and full and fun. Yours will be too, you will be ok.
Don't expect too much of yourself, you have had an almighty shock and you need to give yourself time. Just all be together and there for each other, their love will get you through.