We all figured it was something like this. As most have noted, no groundwork laid for this, no hint of reluctance or nervousness about performing publicly or with others when approached about playing previously, no mention of past trauma or issues with family (no mention of anyone's family at all to this point iirc), including in Kurose's own head, which the manga gives us some peeks into. There's at least a reason being given now, but I still say this is an unearned tone shift that was handled poorly.
But i'll also leave it at that- bro clearly has decided part way through that he wants to write a slightly different story than the one he started. Hence the new characters and shift to a longer, bi-weekly publication. I'm still down for it and interested so let's see what's cookin going forward.
EDIT: I was way off- you do see Kurose's sisters at least twice, once when they help dress him for the date, and again when he's moping after Haimiya tells him off.