I guess it's because the mom assumed like any child, the younger brother was influenced by the older brother, because of the monkey see monkey do idea, so I think that's why she hated the older brother, she believed it was he who made the younger one say those things. Besides at time parents favour one child over the other, in this case it was the younger one. And it didn't help that the mom lost her husband and starter becoming obsessing over the younger one who looks like the husband.
it's because will is the father's replacement for the mother. because of that, to the mother will can do no wrong as he's idealized. and also probably the mother is jealous of of nico and that's how she retaliates.
This is my take on it: younger son looks like dad. Dad dies. Mom loses her mind. Younger son must stay with her and love her the most forever since he's dad's replacement. She considers Nico as a rival for her good son's affections. Well at the end of the day it just loops back to mom went nuts
I don't get why she started to dislike the older brother since it's the younger brother who said he wanted to marry Nico? I feel like it would make more sense if it was the opposite... that she started to dislike Will...
Anyway still very sad... and she's awful ><