Careful Now... Down With This Sort Of Thing.
Seriously, unless your brother was visibly traumatized, is it really a big deal? Your mother/father walking in, I'd say this is bad. But heck, I walked in on my dad very much intentionally watching lezb porn. I shrugged it off.
If people wanna watch porn, that's their thing. Crimes only involve other people. A victimless crime, is by definition, not really a crime. And no, it's not true watching stuff inspires mimicry, I've watched stuff like this for years, and far from a serial rapist or something, I'm still very much a virgin.
"You'd feel differently if it was your brother watching porn." Betcha I wouldn't. I'm uncle to a newborn. If my sister started showing him tentacle stuff, I'd be perfectly okay with it, except for the likelihood of him turning like Yuki from Menage a 3. Not that I'd deliberately do it, but I'd not even blink about content ratings. I saw my first adult film when I was like 8 (in the theatre no less).