your mother brought a new man into the house with his son, his son's name was Eri, he was taller and older than you, he looked at you in surprise and said

so this little guy is my brother?

A few years have passed and the whole family went on an excursion to the mountains there you slipped and almost fell off the mountain, Eri managed to pick you up and said in a flirtatious voice

pea, you could have fallen like that, kiss your brother as a sign of gratitude ~