AITA for intercepting my sister's mail?

Apologies for the misleading title, she asked me to.

Background: I (m22) live at home with my dad as I am a new college graduate who doesn't have enough money to move out (yet). Both of my younger siblings have moved out already. My sister asked me to take any mail with her name on it into my room for safekeeping as she doesn't trust our dad with it. A couple years ago my sister was a victim in a crime, and after the court settlement and insurance she still owed about $900 USD. Our dad paid for it because that was the best option, and applied for a state program that reimburses victims for things that court settlements didn't on her behalf. As such, he has been paying attention to the mail because he expects a check in the mail that will cover what he paid. The catch is that the check will have my sisters name on it and he won't even be able to deposit it without her approval anyway.

My job lets me get home roughly 30-45 minutes before my dad does, and so usually I'm the one who does the stuff that gets done after work. Mail, let the dog out, etc. This past Saturday, I grabbed the mail and there was a letter from the state for my sister. I brought it into my room, but not before my dad saw it and asked what it was. I sent a text to my sister letting her know about it, and she asked me to just open it and send her a picture of the first page. I did, and then put it in the safe I have. Soon after my dad came into my room asking for the letter, to which I said, "No, it's *sister's*. It's her business and her mail." This got him pretty angry, because in his mind he is entitled to that money. The kicker? That's not even what the letter was.

He and I got into it pretty bad. I brought up how he has a pretty fundamental misunderstanding of how the mail system works, and that set him off. He has decided that I'm no longer allowed to touch the mailbox, and he will bring me my mail. I have no intention of following this order because it's simply unfounded and even worse, I don't trust him. Still, AITA?