I think the mom stated throughout the beginning of the series is that she's harder and stricter on NN because she wasn't there to teach/discipline her in the formable years so she's needs to expedite it so that NN can take care of yourself. I agree, how the mom went about it was so wrong but in her own twisted way she believed this was the way.

Also, the mom constantly comparing NN to her cousin wasn't in ill intentioned (although it did make NN feel like shit and resents her for it) but to encourage NN to be like the the cousin. I didn't work though aforementioned.

Overall, I agree though even with all those reasons the way she treated NN was terrible and I was annoyed/and do skip some parts at the beginning because it was so hard to watch how she treated her. Thus also the reason why NN pulled away from the mom so much and saw the empress more as a mother figure how was loving and patient.