What if ML also wanted to confess to FL on that day but he saw her sister first. And her sister probably knew he had a crush on her when he was young and wanted to try this "if you kiss me and doesn't feel anything special then I'll agree that you indeed isn't using my sister as my replacement"...?
Had this feeling her sister isn't a bad person and was just trying to protect her since she probably doesn't view romantic relationships in a good light. Like how she rejected advances from other guys and how their family doesn't have a dad.
