If I caught my brother having sex with another guy, is he gay? And if he is, should I tell mom & dad?

Why are you trying to catch your brother having sex? How about you not do that, okay? As long as he and his partner are safe and respect each other, you should file this in the “none of my business” section of your brain. And FYI: your brother finding other men attractive puts him in more than one possible category besides gay.

If your mother and father are bigoted and would throw him out of the house or disown him, definitely keep this to yourself. It’s not an accident that a large percentage of homeless teenagers are QUILTBAG. If you don’t want to see your brother end up homeless, I would proceed with extreme caution. Don’t out your brother against his will. The sociological ramifications impact him more extremely than you.