It varies. This game has a point system. If you spend enough time with your family everyday, you can keep Deon and the gang’s influence low and your family won’t have sex with them.
If you decide to spend a lot of time with the Mother and Older Sister, but ignored the Younger Sister, Deon’s influence on your Younger Sister will grow to the point where she will eventually have anal sex with him.
As for the futa she is unavoidable. She is your Older Sister’s girlfriend and you need to have sex with her before you can have sex with the Older Sister.