Oh my god, Kawaru, you can't just ask people crows why they're white.
Anyway, while sister's methods are questionable, I think her general goal of "protect my sister from herself before she ends up like dad" isn't dishonorable. It's easy for Kawaru and Pyon-chan to go "We love each other and will work through this and be together forever!", but they're 15-16 and the world isn't so gentle (even in a light-hearted comedy manga). While she may seem to just be meddling (maybe later chapters will delve more into the sister herself), this chapter could be viewed as more of a trial-by-fire: if the couple can't even handle things on the level of high school rumors, it'd be best to call it quits now.