No, not every game does. This is an incest game, so if you're looking for some conflict or something to overcome, there's all the challenge you need. How do you convince family members to go against society and their own initial feelings and have sex with you? The "antagonist" is the incest taboo itself, added on top of the difficulty in seducing any girl in the first place. That's not to say that you can't have an antagonist if you want one, but even without incest you don't need that. You just need a good story, and some challenges to overcome. Believe it or not, challenges can come in all types, and not just some guy that is trying and succeeding to have sex with your family.