In short, his sister flirts with yandere tropes like violence as a defense mechanism and she is better described as a tsundere. Rinko, instead, is a real yandere.

She catches him out running in the morning because his sister hypnotized him into dedicating himself to training to win a relay race. So he has to train to keep up the façade of her hypnotism working. Rinko finds him, and they she really starts to drop hints she likes him. Aki sorta deflects and says he will support her romantic endeavors. But, she immediately gets mad and starts crying. He thinks she is in pain and carries her on his back to his house and puts her in his sister's bed because she won't be home. He believes she is sick and leaves the room to get her something to make her feel better.

When he comes back to give her a drink to help with her 'fever' she tackles him, straddles his crotch, and begins to undress, telling him she will do anything for him. He deflects again, out of concern that it would make his sister sad and she is important to him.

So check this... Rinko then literally, quite literally, begins to violently pound his face in anger until he is busted up and bleeding. Then she kisses him and giggles about their first kiss tasting like blood. Turns out that she snooped around in the room and found the hypnotism book. She figured out everything. They have a long discussion that ends with her more or less blackmailing him with a threat she will blow up the whole situation.

She sets up a scenario to force him to hypnotize her to keep her from causing any trouble... he does, but when he does it, he phrases it in a way that she takes as asking her to pledge her undying devotion to him for the rest of her life, which she eagerly accepts in a panel that is an homage to the iconic Mirai Nikki panel. You know, this one:
https://wompampsupport.azureedge.net/fetchimage?siteId=7575&v=2&jpgQuality=100&width=700&url=https%3A%2F%2Fi.kym-cdn.com%2Fphotos%2Fimages%2Fnewsfeed%2F002%2F053%2F388%2Fa62.jpg

From there, well... he now has to juggle trying to keep her satisfied while preventing his sister from finding out, while she pretends to be hypnotized and uses it as a cover to be directly seductive with plausible deniability.  Rinko more or less steals the spotlight.