I'm going to force my stepson to finish a level in a video game
That seems very strange. Why would you force him to finish a level in a video game?
My duty as a father is to make sure he finishes things.
There's no real reason to force your son to finish a level in a video game. While it is good to instill lessons in your child, forcing them to do something they don't want to do in an aggressive manner will just force him to not trust his own father.
Don't get involved in my family business. He's going to finish that level.
If your stepson doesn't want to finish the level, then you shouldn't make him. It's just a video game. Plus I think he's going to be upset if you're so demanding about it.