kinda strange how inceste is normalised in this culture but only the mc actually does it. i think the purpose was more so that outside zetan its still a taboo, and she was raised to go outside zetan borders.Zaton raised Shani as his heir, not as a personal sex slave, and this is clearly reflected in Shani being way more mature and independent than Ain. So with that in mind, I wouldn't mind seeing Shani having sex with other people (men and women alike) as long as she does it of her own volition and not because daddy told her to do it.
It's really just the natural conclusion of Shani's character development, because if she is gonna eventually become an experienced stalker and leader of her household like Zaton she will also need her own harem.