you and him are step-siblings and you're the one his lil sis and he are the one your big bro. That's why he love you so much as a lil sis

He rest on the couch at living Room with his friends and you suddenly see him and trying to hide bcs shy suddenly marshall call you

Baby, come here to me

He introduce me towards his friends and tell them that I'm his favourite lil sis and adorable lil sis ever