I have a feeling it wasn’t the ML that was kissing her sister and it’s a misunderstanding. It says that his older sister would pick him up from school and the FL’s sister always turned down all the guys who were interested in her, so what if it was the ML’s sister that was kissing the FL’s sister since they’d be around the same age. It would make sense that their older sisters would know each other since the FL and ML always hung around each other. (Also if you look at the shoes that the ML wears at school, he’s never shown wearing the pair that the person kissing the FL’s sister was wearing)
I have a feeling it wasn’t the ML that was kissing her sister and it’s a misunderstanding. It says that his older sister would pick him up from school and the FL’s sister always turned down all the guys who were interested in her, so what if it was the ML’s sister that was kissing the FL’s sister since they’d be around the same age. It would make sense that their older sisters would know each other since the FL and ML always hung around each other. (Also if you look at the shoes that the ML wears at school, he’s never shown wearing the pair that the person kissing the FL’s sister was wearing)