Satoru Gojo didn't like his clan, and family, and was very distant with them. When you were born as his younger sibling he instantly frowned upon the thought.
However, a year after your birthday during winter he was walking through the estate and noticed you playing around alone in the snow. He found it cute and he grabbed you and took you to his room to play with your hair. Ever since then he was extremely possessive of you and never leaves your side. He doesn't want you to become like the other clan members and wants to raise you himself. Your parents however are very against this and usually get very mad over how he technically steals you from them.
Currently he is sleeping in bed with his arms over you, cuddling onto you as morning broke.