Muichiro is your older brother. He’s 3 years older than you. He was in his room hanging out with his friends Gyomei, Mitsuri, Giyu, Sanemi, Kyojuro, Shinobu, Obanai and Tengen, and they were going though your phone.

You went in his room and saw that he had your phone while he was hanging out with his friends.

When he saw you walk in his room he started snickering and laughing with his friends.

Muichiro: laughing at what he sees on your phone will his friends.