Who was the father? (spoilers)

I wish the movie had been more clear about who the father of Peggy's baby was. There are a couple of obvious suspects:

Chris (her father) -- The obvious suspect. Besides the fact that abusing his daughter would only be the cherry on the ice cream Sunday of horrible acts, when he is raping the Woman he lights the room with a candle, and the same style of candle can be seen in Peggy's room. In addition, he definitely gets angry at the teacher when he thinks she is accusing him of being the father and kills her so she can't spread the story.

Brian (brother) -- It's telling that when the teacher tells him Peggy's is pregnant, his first reaction is, "She doesn't have a boyfriend. Are you accusing Brian?" That's not really the demeanor of a guilty man. He eventually gets around to himself, but his first reaction is telling. You could also see his insistence on killing the teacher being a "protect my son" reaction.

The Boyfriend -- Early on, we do see an interaction with a boy. Peggy is clearly avoiding him and hiding the relationship from her father, and near the end of an early scene we clearly see that Peggy has spoke with this boy and he is NOT happy. In addition, we hear that Peggy was a normal, happy-go-lucky girl who dressed provocatively until a month ago. Yet she is far beyond only a month pregnant (according to the teacher, she is starting to show and women typically start to show between three and four months. So any incest would have occurs many months before, and we would expect behavior change to occur as a result of the incest and not upon discovering you're pregnant. Peg's behavior makes much more sense as a girl with a psychotic father that she is terrified of finding out she is pregnant as opposed to a girl who was abused by a family member.

So I think that the evidence is clear -- the father of Peg's baby is the boy we saw at the beginning of the film, the boyfriend that Brian referenced but was hidden from Chris.