Hmm, it would be possible, though limited in cases if one to go fully into such a mod.
Wouldn't work as an addition to the FFFox Mod, but could certainly become a separate side-mod later on down the road. My current focus would be on adding in DLC compatibility and a few additions afterward, but we can see for in the future. Technically with the addition of the Languageset for the FFFox Mod, one could switch up how the sister talks and brother reacts to make it much more sister active and brother passive if anyone is up to the task.