You can't blame the mother. If you we're the mother, of course that's just disgusting and immoral between siblings even as she thought that Yuzuru is blood related to Ai's father which she thought they are half siblings. The father is the only one who knows where Yuruzu came from. Anyways this is not INCEST although Yuzuru's parents are in fact incest and he is the product of it.
True Love (sugiyama Miwako)