Future planned content: Sex with the girls (obviously), NTR (Devon has sex with them), Share (You can share them with Devon, or he can share them with you)
You can determine your relationship with the characters at the beginning of the game, to enable incest answer the questions this way:
She is your - Mom
Her late husband was your - Father
And you are her - Son
Now this girl. She is your - Sister
And you are her - Brother
Nickname for her - Sis
Nickname for you - Bro
Finally this guy. He is your - Brother
Nickname for each other is - Bro