Both minimum and maximum stats haven't been an important part of the game for a while now. For example, during the topless team contest, everyone was acting like a full exhibitionist. And many characters have begun to change their behavior without any stat change. I think this is fine, mainly because everyone in the main group is getting along now, and no one is confused about what is happening around them. It makes sense that a girl coming long later wouldn't need to go through a slow transformation process, since they're just joining an existing family, and would be able to figure out how they feel about that without as much prompting or intervention. (They would still need new experiences to transform.)
The way I would wrap up the stat system, without needing to do any additional work, is to have a single cathartic moment for the still developing characters, (such as what happened with Sarah recovering the horse) in which they acknowledge their new feelings/relationship/status with the MC (whatever that means or requires for that character) and then change the entire stat bar into a single word or title, summarizing what their role now is. This title could be amended if they continue to transform.