Jeez Fianna, it's your bloody computer. If my little pervy shite of a brother was visiting porn sites on my computer I'd tell him to go out and get laid. Quickly.
Also, surely he can afford his own computer?? He's taking full advantage of your sweet natured generosity and personally, there's no way I'd stand for it. Yes, it's normal to have physical urges. But it's hardly normal for an older brother to use a younger sister's laptop to visit porn sites. Really now.