She and Makoto seem to have a good relationship, implied in Monokuma's motivational videos. In the video it showed Komaru and her parents cheering her brother on when he succeeds to enroll in Hope's Peak Academy. They also have a similarity as siblings as they both have the same ahoge. According to Komaru herself, the two watch TV together, as well as share thoughts on manga they share with each other. She's also mentioned that they did argue quite a bit which is rather common in siblings.