I hid my step-brother's drama portfolio and he ended up failing the course
Why would you hide his portfolio?
I thought it would be a funny prank.
Hiding your brother's portfolio was an awful thing to do and you should feel horrible. I would tell him what you do and apologize profusely in hopes that he will be able to forgive you.
He left out his portfolio in the middle of the night, I had to hide it.
You need to apologize to him and tell his teachers right now. He should not have failed because of you. It was wrong what you did.