My prediction is that the mc will father himself and his sister, and father children in the past with stepmom, grandma, and best friend's mom like he did with the aunt.
The game specifies how time travel works in this universe. The MC can only go back to specific points in time and change things, but doing so alters the timeline. This makes it impossible for the MC to be his own father. He could only travel in time if he was already born. There is no way he could be his own father because he wouldn't exist to go back in time. It would be like Chloe existing BEFORE the MC saved Olivia's life. It just isn't possible.
Remember in the pool the ladies are talking about the night that Olivia almost died and she was the last person to see the MC after they had sex. They jokingly accuse her of killing the MC because they NEVER saw him again after that night. However, shortly after that revelation, he is whisked back in time against his will to after he had sex with Olivia and meets all three of them, which makes what was said in the pool erased from the timeline and a new timeline now exists.
Is it possible he could create new kids with the girls? Absolutely. Is it possible he can be his own father? No.