"Have sex with your kouhai-onee-chan" is one of those sentences I never expected to read.
This line was bothering me a LOT (just like most people here) so I went and looked it up in the raw:
Basically, the translator messed up. It's not like it's easy to localize, but localizing it as "have sex" is DEFINITELY not good.
She uses the word 性春 which is a homonym for 青春; they both sound like "seishun", but the original word which means "youthful experiences" had the first kanji replaced with a kanji that means "sexual". In other words, she's so sex-crazed that she was thinking of sex when she spoke the word, and it twisted the kanji, in a way that nobody can notice because verbally they both sound like the same "seishun".
What he's hearing: "Enjoy the springtime of your life with your kouhai onee-chan!"
What she's basically thinking as she speaks: "Enjoy the sexytime of your life with your kouhai onee-chan!"