Uhm... can we just mention the MC just found out about this AFTER the parents married and went on a honeymoon, and from a sister that's still a stranger, there's 3 ways this could have happened without his knowledge:
1-his dad led a double life up to this point with his son none the wiser he had started a relationship, and didn't even bother inviting his own son to the wedding... or at least making the introductions
2-his father and her mom met that same night, got drunk and married on the spot, the vegas thing
3- MC was so absorbed in his games he literally didn't hear about it when he told him.