Email swap

[Vaan]

So short story: my brother decided to gift me rimworld as my birthday present and used his email, now when I go to play the game I see that:

1) Said email is long gone
2) The Steam account where I used to play games can't be accessed because there is email auth
3) I have no CC of the email containing the digital key of RimWorld

Basically all I have is the email with the link to get the game locally, but I can't access the game using steam as the email is dead, is there any way to prove that I am the legit owner and swap the old email with the new one?