As long as there aren't NTR routes with MC's family members, I'm cool with it. Since you talk about how realistic something is, it'd be really weird for all your family members, that love you and are pining for you, to suddenly/eventually turn to another guy/partner. Especially in an incest game. Miru or a side character is definitely a good way of progressing that narrative. In the original game I was given an option for Miru to be a cheating whore every 5 minutes (exaggerated I know). As it's probably clear, I'm not a fan of NTR, but I believe a path like that needs to be built up and having Katie who loves and adores MC (secretly) would be a dumb NTR path. Haven't played in a while, so may have gotten some points wrong.Yes.
It only happens if you do a combination of treating her bad, making her do it, and making certain decisions that lead to events that can escalate.
(Except you go down the sharing route and its sub-routes. The deeper you go, the more you might get lost.)
I hope I don't see any surprise routes for the sisters and mom D:
As usual, you've always done a good job of creating believable characters (except for old Miru :/ ) just the logistics and actual events were poor at times (camping scene which was broken at times). I can't wait to get to play this new version. I want to wait a little more before I dive in! Much LOVE!