Wish we'd had more on Marianne's family back story. Agree with PP that her mother's character two dimensional
It's not in the novel either. The mother isn't fleshed out, other than her being a solicitor, her snobbery and her strong preference for Marianne's brother, and that she's fine with him using physical violence towards M that's literally all we are ever told and we know nothing at all about Marianne's dead solicitor father, other than that she has a strong response when her nasty boyfriend smashes champagne glass belonging to him in their Italian holiday home.
I think that's a deliberate decision by Sally Rooney -- she doesn't do backstory. Connell never follows up on finding out about his unknown father, even though Lorraine says he only has to ask her, and there's no attempt to connect the fact that the Waldrons are considered a 'bad' family, with several of them in prison, with the warmth and conviviality of the family members who are in Lorraine and Connell's house at Christmas..
It's the same in her previous novel Conversations with Friends -- the main character has estranged parents, and an alcoholic father who sometimes phones her at night and sounds as if he is about to commit suicide, but even though Frances goes back to her hometown to visit both parents, there's no backstory about their marriage or its ending.
I think some people really liked that, that Rooney left out the dutiful excavations in the characters' pasts that slows down the action in the present.
(I have only seen the first two episodes, so don't know whether Rooney changed any of this in the TV scripts...)