The difficulty for them adding it, besides time constraints, since they are close to the end, is that when someone asks you to create something that you are not really in to, your heart is not in it, and you end up doing a half assed job with it.
I used to write custom erotic stories for people. Most of the time it was vanilla stuff that was easy to write, but a few were very extreme things that make incest look tame, that I just was not into, and those stories were a mother fucker to write, simply because I disliked writing them and that meant the stories ended up not being as good as it could have been, if I had been into it.
That was why I made the suggestions above. The pet content is very low, so all they need to do is show mom going into the room with the dog, the writer writes in a couple of moans and maybe a bark or whimper from the dog, and D's thoughts when she catches her mother along with the one pic that infers that the mother is being fucked by the dog.
Then, even if the mother had caught D fucking her father. The pic of mom fucking the dog would be far worse and it could be used to tell the mother to fuck off.
It would need only a tiny amount of buildup, and she could have been with the dog owned by her new friend that she was dogsitting for a day.
Mom gets a call from friend: "Hey, I need to go out of town for a couple days, can you watch my dog for me?
D's Mother: "Sure, no problem."
A few whimpers, moans, barks and clicks later in the story.
Mom bursts into the room catching D and F in the act, she screams triumphantly at D to get dressed as she is going home with her.
D: looks her mother in the eyes then grabs her phone off of the night stand and shows her the pic of her (mother) fucking the dog and tells her to fuck off...
Unlike the last time the mother got busted this way, she had closed the curtains, but not enough. Which is why you only see part of the image with her and the dog, but it is enough.
After the mother leaves, as D is taking her fathers cock into her mouth again, she asks him when she can have a dog of her own.
At that point, D's corruption is complete, and you have seen a new side of D that has the strength to be a stronger more confident person.
Game ends here, or maybe in a pet store shortly after that.
the key here is entertainment, some might enjoy, some might abhor but ultimately its YOUR CHOICE (or mr. dots choice) whether to go through with it, that's all it comes down to...a choice. and frankly it could add a flavor to the main story as well...D likes to experiment now becoz sexz is fun and interesting. you (the dad) did give her some freedom of choice...so yeah---bestiality could be on the scene...she could even do it once in a while, when "dad" is doing "business" with other women 
