I accidently activated a steamgift on my brothers account. EDIT: PROBLEM SOLVED,

Nope, too late now.

Dont download it.
Write Steam Support, they will repack it in his inventory. so you can gift it to the correct account

Or, buy another copy and do the right thing.

Edit: corrected.

You can talk to the creator.

you sir are a good lier!

Steam support will do nothing, they would only remove the game. The only thing you can do is to buy the game yourself and activate it, else you'll get suspended soon.

Next time think more before you activate it on "your brothers" account.

The story has a lot of holes in it

So you were logged in your steam client, but your brother was logged in on the browser, the email that gives you the link to redeem the game would've ask you to log in with YOUR steam account, sounds like something doesn't quite fit.

What's your brother Steamgifts profile?

Write to steam Support (with evidence) That happen to me 1 time when i wanted to give a Present to my gf that i won (it was not a steamgift group if your wondering or in this website) I explain; gave both profiles and giveaway details Screenshots and link. after 1 week they took the game out of my steam and i gave it to her.

that happen cause of my fault cause it was the first time i won + saw a serial number lol, I was expecting to see the name of the game before a question of adding it to my steam account appear (wanted to make sure the key was working) so i could give it to her....... but yeah it just added it auto (Stupid me lol)

PS: Being honest pays

Hope you find a solution soon, the best thing is contact steam support, they probably know what to do. Best wishes

as others already said Steam can repack the gift and you should do that. You should also write the gifter and explain this aswell as SG support team.

PROBLEM SOLVED