Your sister has a boyfriend, in fact she has more than one. But tsukishima doesn't deserve that treatment.... And your sister doesn't deserve him. So who does..... you? Do you deserve him? Do you want him? More than her? Interesting...

She was so different from him. While she was shy and had difficulties speaking her thoughts, he was as outspoken as ever. He was so different from her. When he talked, his voice could be carried throughout the whole school. When she talked, her voice could barely be heard. They were so different from each other, but maybe that's why they were attracted to each other. (Cover credits to @Rozero-San)