I am a little late to say that, but.. it is hilarious how so many people are buying the family friend excuse here.. Have you all really been reading the same manga? Did you miss all the inappropriate shit so far? lmao
It is the same idea as what happened with the first kiss scene. The MC and Kurumi didn't want to make things "unfair", and even if he were to kiss everyone, someone would be first.
It is just more nonsensical bs to add to the comedy, nothing to really worry about.