Why is she saying that them hiding things about her mother was not done for her benefit? She (The mother) wrote a book about how much she hates her children, that does seem like a pretty good reason to cover-up some information on your mum, because you can't tell me that knowing your mother actually despises you isn't going to do some psychological harm.
Also, did the Auntie actually bad-mouth her sister/the mother? It's hard to tell if those were her words or the group's words.