I found out my little brother is gay today

So he's dating a girl in my grade level little brother. The two of them went to hang out after school today which they do that pretty daily but it's not my job to watch him and I just assumed they were friends, they've know each other since elementary. Well when they was walking to there car, the sister came to talk to me saying how cute they were together and if we should be worrying about them getting std's. I was like um what? She acted surprised and said she assumed I kew they were having sex. I was completely confused and after a while she figured it out and apologize and walked away. I talked to some of his friends to see if they know and they did. Some of them said they knew for years now. He's 16 and im 18 so I don't know how long he would have known. He came home today acting like he always does. I'm offended he didn't tell me and kinda worried that their a reason he's not telling me. Were pretty close and barley fight.