If I can raise a baby at 21 then she is emotionally mature enough to realise that her behaviour is wrong.
Not everyone is the same. Not everyone has the same inner resources. People respond to trauma differently, which is what growing up watching an abusive dynamic between parents is.
It is ABSOLUTELY NOT OK that this daughter is behaving abusively towards her mother. At the same time, trauma is essentially at the root of a hell of a lot of mental health problems. It is the product of trauma and I'd probably loosely term it as a dysfunctional attachment.
The OP has casually referred to the ex husband as a 'narc'. Narcissistic personality disorder is a serious mental health problem. Some people do like to argue that PDs are somehow 'different' to the mental illness they see as valid and deserving of their sympathy, but bluntly that's nonsense. PDs are treated under mental health teams because they are mental illnesses.
So if the daughter has a history of serious mental illness in her family, and also grew up in an environment where she watched her dad abuse her mother - it's reasonable to say she is likely mentally ill, and her emotional maturity will not be comparable to an emotionally healthy 21 year old at all.
This is one face of mental health that a lot of people are unwilling to deal with - that mental health problems can result in deeply undesirable behaviour. The OP's daughter needs professional support and treatment.
It's not palatable in our current climate of pretending that #raisingawareness and #selfcare are enough, and everything will be rosy.