Okay, so a bit of background: I'm 15, and my brother is 19. I technically have my own laptop, but I still need to ask someone to put the password in for me (because parents). My brother knows the password, so I will ask him sometimes.
Now, I moderate a Stack Exchange site. (Yes, it is permitted for a minor to be a moderator.) This is not immediately visible to most users looking at this question, because reasons. When you become a moderator on any Stack Exchange site, you must agree to the mod agreement. This says, among other things:
ii. I acknowledge that I may have access to potentially personally-identifying information about Interpersonal Skills Stack Exchange users and that in connection with such access [...]
b. I will not disclose this information to anyone
Now, I will sometimes leave pages open on my computer. These include mod pages, which can sometimes include PII if I'm, say, looking at a voting ring. Sometimes, when I have him log on, he'll page through my tabs... and try to sneak a look at the Teachers' Lounge or the pages I've had open. He has not looked at any pages with PII yet, but he has taken a look at the (cleared) flag page. If he looks at PII from my account, that seems like a violation of b. in the mod agreement. Thankfully, this hasn't happened yet, but if this doesn't stop soon it might, and I really want to avoid that.
My parents know that I mod a SE site, and I've told them that I am really uncomfortable with my brother looking at stuff on my computer. They refused to talk to him about it, saying that a.) He's not going to do anything, what's the problem? b.) I spend too much time on there already, so they don't really want me bringing it up again.
I've already asked him to stop a couple times, saying that I really don't want him looking at the stuff, even if it isn't PII, such as the Teachers' Lounge chatroom. He hasn't so far (keeps trying to get me to press keyboard shortcuts and stuff that will switch to my mod tabs while he's watching, but I'm not so tech illiterate that I don't know what ctrl 1 does). He's relatively familiar with SE - he has 20k rep on Stack Overflow, 14k on another site, and 10k on Meta.SE, so he's not unfamiliar with the system. (Hi. Yes, I know you'll probably be reading this question at some point. Maybe you'll take me seriously now...)
We're both culturally American, although not living in the States right now.
This isn't just about the mod stuff - while that certainly contributes to my unease, I don't like him watching over my shoulder watching what I'm doing generally. (Or looking at my phone.) So:
How do I bring up the issue of my brother reading what's on my screen?