Why nobody liked Edith?

It was mentioned in another post that the entire family treated Edith badly—her parents didn’t expect much from her or pay her any attention, Mary was cruel to her all the time, and so on.

Though all of this is true, I believe there is a reason behind it. Edith was never kind to anyone purely out of kindness.

We know that when Sybil died, all the servants had something good to say about her—how she supported them, how she spent so much time helping Gwen even though no one asked her to, and no one expected her to.

We also know that Mary helped Anna a lot during her pregnancy, provided a bedroom for Anna and Bates, and even tried to cover for Bates when she thought he had killed a man.

Edith never did anything like that. She only helped at a farm, but she did it for fun and ended up kissing someone else’s husband, so I cannot consider that an act of kindness.

So, was everyone wrong treating her the way they did?