That's a great theory! I feel like I shouldn't give away the answer, so I'll stay quiet for now... The truth might become clear in future developments. (Though I'm still brainstorming the plot, so nothing is set in stone yet!) Please look forward to it!
The story presents two possible interpretations of the sister's actions:
1. The sister is naive,She doesn't fully understand that such sustained knee-to-groin contact would inevitably cause a physical reaction, acting on only a half-knowledge of the male body.
2. The sister is deliberate, She knows exactly what friction will cause, and uses "self-defense practice" as a pretext to get close. Her real plan is to knee him hard if he gets aroused, which is a small act of bullying(playing) or just want to get closer because of his avoiding.
Both dynamics are understandable. Personally, I'm more intrigued by the latter.