I Love My Sister's Boyfriend summary:

Fateful Encounters:Judo-loving Mitsuka has been looking forward to finding her true love since starting high school. While walking through the park one afternoon, she encounters a very cute boy napping on a bench. The next day a new coach arrives at Mitsuka's judo club. And who turns up? The cutie on the napping on the bench of course! His smile makes her heart race, and her name on his lips makes her face blush. But there is just one minor hangup....he turns out to be her sister's boyfriend. What's a girl to do?