They are adopted

I m quite sure they arent really the offsprings of the couple. Though its clear that they do care for those kids but in their own twisted ways. Thus its not incest since the sisters and the brother arent related, the father prob feels that it will be better for him to have sex with an outsider than with his 'sisters' that he grew up with. Though the point of the sex is a mystery.



There was a scene where they mother told them she would give birth in a few months to a brother and sister and a dog , but if they behave she wont do it except for the dog. The mother doesnt seem pregnent at all and its also not possible for her to decide on the sexes of the twins that she would give birth to and u cant just terminate a late stage pregnancy. Thus it must be adoption she is talking about and so its very likely the sisters and brother are as well.