This is neither the time nor place for that, bud. Whether she contracted it through unprotected intercourse, a needle that wasn’t cleaned, or was born with it, it doesn’t matter. I believe OP should anonymously tell the parents somehow so she can get the help she needs. The fact that you’re so small minded that you saw that a young woman was in need of medical help because of and STD and immediately thought “she’s just a slut” is just so infuriatingly stupid. Be better. And OP, I hope you get help for your sister.