Responses
Vol 5 ch 4.1-4.5 is the first spinoff, and then vol 7-8 is Misaki visiting Nagoya with the finale between Kunihiro and Misaki being vol 9 ch 6.2. Kunihiro returns in vol 15 ch 3.
Kunihiros first appearance is vol 2 ch2-5
I have been reading this one for so long I can't remember from which chapter is the brother story