Incest isn't really my thing, but I find myself very accepting and appreciative of it in this game because all of the family members are friggin hot. I end up skimming past most of the dialog at this point anyway.
I love the numerous available endings, the gorgeous models, and the fact that you can go through the game without actually doing anything incestuous. Skipping the incest is fairly difficult and cuts the game content drastically. Still, loads of sex happens either way you do it.
On my best playthrough, I had sex with 18 different women. I ended up marrying six of them and having babies with three. I was left wondering if the genders of the babies was setting up the sequel and how the mother from the first one would look at that point if they did.
Unless I am incorrect, I don't think it is possible to have sex with every character on every playthrough. I believe this is directly affected by your relationship with your mother and which path you choose to begin it.