I was partly responsible for giving my brother an STD?

I have sex all the time and I always use a condom. I had a threesome with this guy and my brother's girlfriend. I gave her the STD during another threesome weeks before and I guess one of us gave the dude the STD. Anyway, when my brother had sex with his girl, he got it cuz she gave him oral without a condom. He's blaming me for giving it to him when it was his gf's fault. Should I be held responsible for giving him the STD??? BTW, it's a curable one and all we need to do is take pills for like a week or so. I'm 19 and they're 20