You are right, incest games are highly unrealistic. Personally, I see incest this way - you are 10 years old, you masturbate and cum for the first time and decide to share this new pleasurable discovery with your sister. And by the time you are 18, you already have a daughter or two who goes to the same school as you.
With your mum it's a bit different. You are 6-8 years old and she bathes you, at the end, as usual, she talks about hygiene and starts washing your cock, her tender motherly hands gently soap it and wash it off and you don't notice how your cock starts reacting to her touch, the usual water treatment becomes a pleasure and you cum for the first time in your life. Your mother looks in surprise at the white liquid in her palm and at your blissful face. She loves her son and wants to make him feel good again.
But no way, you turn 18 and suddenly decide it's time to start fucking, I don't believe that living with the hot sisters and mum you could last that long.