Also, speaking of relationships - the idea is not be connected to everyone in the family already when it comes to incest. One or two is ok - parents are more then likely to have the most at the moment as they're incest with their siblings already (the other mother/father) and perhaps one of their own children too.
Meaning mom 1 and dad 2 are brother and sister and probably have played together
mom 2 and dad 1 are brother and sister and same thing. The four adult siblings possibly have gotten together without the kids over the years as well.
Now the intent is to bring the entire family together.
I would imagine it is more likely that any play within the respective families has been a parent to child intro to incest. This wouldn't be the only time the two families have been around each other but aside from Alice and Abby and sometimes Izzy playing as young girls together, there's not much between the cousins yet.
MP, please correct me if I am mis-stating any of this.