Paul and Eve are supposed to be married. But the relationships between other characters pretty much is a mystery, as developers never has said anything about it. I always say that Emily doesn't feel like a blood-related character because of the way she has been written, so my guess would be that she is supposed to be either stepsister or cousin. Paul is pretty protective about her, and since he is such a dick towards MC, Paul could be this stereotypical stepfather for MC and father to Emily, this way making Emily the younger stepsister for MC. Eve would then be the mom and since you can't even choose the relationship with Kiara, I would say that she is meant to be like the blood-related older sister for MC who shares the same parents with him.
BUT, it could also be that Eve is MC's aunt, which would mean that Paul would then be aunties husband aka uncle for the mc who acts like a dick and protects his daughters, meaning that Emily would then be the younger cousin for mc and the youngest child of Paul and Eve, and since you can't choose the relationship for Kiara, she would then be the older cousin of the mc, older sister of Emily and the first child of Paul and Eve.
Since you can set the relationships any way you possibly want, Eve could also just be set as the owner and Emily could be set as "owner's daughter" or something like that, so it's not necessarily incest which then means that the incest tag isn't added for this, as it hasn't been confirmed if it's really incest or not. I would bet that it's meant to be some kind of incestuous relationship game as Inceton Games is involved in this, and since they give you the opportunity to choose couple of the relationships, but in the end you decide the scenario and see how it works out.