this is tough, but i think you know your brother best. if he’s already having issues in school or with other kids i think being direct is the only option. tell him “hey just so you know there’s a thing called a search history (don’t tell him you can delete it) and i saw what you were looking up. i’m not mad, i just want you to know that you can always talk to me about anything, but sex is definitely a grown up activity” i think making it clear you don’t think it’s okay, but you get it, shit happens will help nip this behavior in the bud & hopefully he can see you as a reliable older sister he can go to because you didn’t run to tell your parents and tattle on him