I don't know what to do with this.
Why is it notable that your sister is Mexican? Aren't you Mexican, too? Is this your subtle way of ruling yourself out as a possible source? Like, my sister is Mexican and I'm not, so if she has the clap, I can't possibly have it, so there's no way I could have given it to her during sexual intercourse?
Look everyone has sex with their Mexican sister. It's not a big deal. Not cool that you didn't let her know that you were diseased, though. You probably should have worn a sombrero.
Short Answer: STDs aren't the end of the world. Almost everyone has them. My wife has almost all of them. I have none. Isn't that weir-wait a second!?!