Not sure, but I think it sounded like she was nice toNico until she figured out he was gay and then she saw him as a rival for Will who she seems to confuse with the dead father.
That also makes sense... Though no matter how much I rationalize the why, I still can't wrap my head around the mother's behavior being acceptable, even among the whole incestish thing. I mean, they were young and she went away overboard.
I Think we can also all relate to favoritism and neglect and how much it sucks so I think this concept really makes us (me, if no one else) get riled up. Totally agree though, she totes sees nico as a rival in her unhealthy attachment to Will.
I have this odd theory or idea that the mother thought she was going to be unable to have children or married with a previous child from another marriage, and that was Never co. And then she either treasured Will because he was her "real" child, or reminded her the most of her late dead husband. It would make sense that that was why she was so nice to Nico until Will was born. Or she is just a witch with a b. Either or.