Ok is this child abuse?

So my brother and sister in law have a 6 year old and she isn't in school and is NOT potty trained, her mom doesn't bath her but once a week if that, she does what she wants when she want, they live with my grandparents and my niece hits, kick, spit, screams at them, and they can't do anything cause her mother said if they do anything to her she would take her away from them and they would never see her again. I think it's disgusting that she 6 and still in diapers, and her mom doesn't care, she just lays around all day doing nothing, they don't pay any bills, my grandparents are well off but they worked hard for what they have and don't mind helping them, but they have lived there for 8 years now and never offer to help them out.