The author needed a villain so she went overboard with the mom and made her totally unreasonable. Some resistance is to be expected but that's not what happens with this story (I have an otakumole account, I already know what happens). She's basically a Yandere mom if there was such a thing. Her facial expressions and the way she clings to Ai creep me out. （￣へ￣）
Oh and I'm not sure if she actually knows they are not related but she goes way overboard anyway. In the end (unless I missed a page or something) it doesn't even look like they reconcile as a family either.
She knows they aren't related.
Oh..yeah she's definitely a b1tch then. (￣∇￣")
because the dad let her think Yuzuru was his, so the mom have been thinking they are half siblings all this time
That is the only logical explanation.
If I remember correctly or correct me if I'm wrong, I think they were already married a little bit before the mom saw Yuzuru.
If the mother knows that her children are not related why is she being a lunatic??