Rereading 6.9 after reading 14.5. I think you're right.Mother Daughter incest is a possibility. There's defiantly something that the two are hiding. There could be a physical relationship between the two that they're hiding.
Although I wonder with the newer chapters if the mother is now sterile, part of her mysterious illness, and the daughter plans on having her kid like being the "real" wife?
I don't know this series is wild. It could be anything. Many things are hinted at little in the way of firm evidence.