Are they twins? just very close in age?
According to the manga adaptation
___ Blood siblings born less than one year apart, which is why they are in the same school year. Their last names are different because their parents divorced and one went to live with the father and the other with the mother.
wjs018@ani.social 6 months ago
Thanks! This makes more sense now. Also, from fishing around in the manga to compare some panels against the show, I ran across this explanation earlier than where we are in the show. So, I assume the explanation was moved to be revealed later.