Is this a fact that Ani stated themselves or just your theory? This makes no sense. First, the doll quest only removes your sister from the game after you have completed it. Before that, she still exists as a character in the game. It literally doesn't solve the problem of having underage characters in the game. It even puts a bigger focus on the sister.The change happened around the same time when the Corrupted Kingdom patreon page went under review due to the presence of kids in the game. No sexual interaction happened with said kids, they were the children you had with each member of the harem and you could visit them at the daycare or go out with them and respective mother for some wholesome time. But patreon said no kids in a porn game and blasted the dev for that who only had 2 choices: leave patreon or remove the kids.
That move made some dev rethink the mere existence of underage character in their game even if they had no plan to sexualize them. For Ani and futa family,the dev chose to make the little sister disappear with the doll quest making her dead in the story and th MC seeing her as an hallucination.
Not to mention that you can also make children in this game. Heck, you can even pay one of them to kiss you.
Also, the quest ends with "Continues in a future update" or something like that.