nah.....

Yes, She is. I can name three instances where She raises Her voice, to Her Grandmother. 1, when Violet, disagreed about the House being used as a Convalescent home, and expressed her views, in which Sybil, yells Why! Another was when Violet had expressed concern about Sybil's choice in the wrong sort of man, She may end up with. and when Sybil and Branson declared their love for one another. Violet was trying to tell her she was making a mistake, and Sybil,snaps at her. Robert angrily told Her not to speak to Her Grandmother that way. She's a brat!

So she voices her opinion everyone once in a while to her oppressive grandma it's not like she leaves the room when her Grandma is telling her something. Sybil had different views and she has to keep them bottled up because she has no one to talk about it with, so when old Violet comes along and angers her she's gonna burst a little. Mary up and left when Violet made a crass joke after Pamuk's death and I don't see anyone on her case

not more than other teenagers

How is she disrespectful to Violet? She has her opinions, and she expresses them. Violet does the same.