The cannibal dynamics

It's been a long time since you asked this, so maybe you aren't curious anymore. But here's what I can offer (SPOILERS GALORE...)

1. The blonde woman who was whipping herself was not well explained in the movie, but it was in the book. Basically, she was supposed to be supervising the younger children while they were out on a "hunting" expedition. The young kids got out of control and killed the baby that we see in the plastic bag, which was a "bad killing" by their standards. Apparently there is something about spilling/drinking the blood of the baby that is supposed to give them power, and since the kids got out of control and killed the baby ("in its sh*t and p*ss") without drinking the blood, the power is wasted and the spirit of the baby will curse them. So the older girl knew she had screwed up by not supervising the younger kids. So before she returned home, she whipped herself to show that she knew she was bad and to punish herself in hopes that it would minimize the beating she was going to get when she got home. The movie didn't really follow up on that so it was useless to have it in there at all. In the book, all the kids (young ones and older ones) got beaten when they got home.

2. Yes, more or less. They are all descended from the same 2 people: the daughter of the lighthouse keeper (alluded to in the newspaper headlines in the beginning) and another child who was stolen/lost a few years later, although they would occasionally "steal" someone to breed with to bring in new genes to the pool. That is what that one guy was there for, the guy in chains who was clearly mentally ill at that point. In the book they called him "The Cow" and his only role was to mate with the women when they demanded it and be abused by everyone else.
3. See above about The Cow. Not sure how long he had been with them, but a pretty long time. He was pretty much gone, mentally, after so much torture and rape. They never explain what he was like before they kidnapped him, or how they came to have him.

4. This is a complicated question, because it's different in each situation. They wanted to keep the baby, Melissa, alive only until they could get her back to the cave and kill/eat her to counteract the curse from the baby that the kids killed the "wrong" way. They kept the mother of the baby alive in hopes that she would lead them to the missing baby (Melissa) because the boy had run away and hidden her. They didn't intend to keep her alive after that. Same for the dark haired woman, the friend. They were just there to help the cannibals find the baby. Quite honestly I don't think they kidnapped that one woman to provide milk for their cannibal baby--I think they just made her feed the baby as another means of torture. In the book they had no emotional tie to their baby, it was just there and they pretty much ignored it most of the time. They just made the woman feed it because they could tell it was going to freak her out.

The reason they kept Steven alive was another storyline that was in the book but was sort of dropped by the movie: Steven was really dark and evil and capable of really terrible things--as the scene with the hitchhiker hinted. In the book he had also murdered a woman that he had been having an affair with, and he was fantasizing about/planning on murdering the hitchhiker too. When the Woman (the lead cannibal) met him, she sensed that he was capable of really terrible things and had a really dark strength/insanity in him and she thought it might be interesting to try to keep him and convert him to be a member of the group (through torturing him into submission), sort of as an experiment. (ironically, sort of like what happens to The Woman in the next movie when she is captured by the father of the family to see if he can teach her to live in our society by repeated rape and torture.) But she doesn't have time to deal with him when she first meets him, because of everything else that was going on at that moment, so she "hamstrings" him and comes back to get him later. However, when everything goes sideways and she realizes she is the last surviving member of the clan, and she encounters him as she is leaving the cave with The Cow, she is extremely weak with hunger and thirst and also needs to gain some strength to combat the curse that was still obviously on her from allowing the baby to be killed the "wrong way" so she decides to kill him and consume his blood while his heart is still beating, which is the way they gain the power from their victims. (wow, that is a long sentence!) So that's why she ended up killing him and eating his brain instead of trying to convert him.

I think the movie tried too hard to include everything from the book, and it struggled to explain things clearly because much of the book is describing the inner thoughts of the characters, including some cannibals who barely speak. It was not a great movie. Of course, the book wasn't all that great either--though it was extremely gory.