Harriet Shelley died aged just 21, after being hit by a car in Bristol.
Returning to social media for the first time since her death, former Union J singer George said he would “never accept” his sister was no longer with him.
Sharing a photo of them together on Instagram, he wrote: “I will never accept that you’re gone. You will always be with me.
“My best friend, my sister, my angel. Too pure and too beautiful to walk among humans, you now exist somewhere else, and I am honoured to have had the privilege to spend 21 years with you by my side.
“Don’t get used to it up there without me, I’ll see you soon beautiful girl. I love you Harriet.”
Harriet had been in intensive care since her accident, but her condition worsened, and a friend confirmed she had died on Saturday 6 May.
After news of her death broke, George’s former Union J bandmates Josh Cuthbert, Jaymi Hensley and JJ Hamblett were among the first to pay their respects on Twitter.