Thanks for the two replies posted.
My husband and I have discussed the situation at length. He is very hurt and now very angry by his step daughter's behaviour, (let's call her G) as he has supported her through many issues, and other relationship breakdowns. My husband and I both have our own health problems; my husband has only just recovered from cancer. G provided no support or comfort to him during his illness, when the treatment left him fatigued and emotionally vulnerable. During this difficult time, I remember several sleepless nights, where I held him while he sobbed, after arguments with G.
G appears to us to be completely ungrateful and selfish, constantly admonishing us for not sufficiently supporting her. Apart from narcissism, she tends toward passive-aggressive stategies like sending hateful text messages instead of discussing things openly. She rewrites history, inventing offences where there were none. She has a chip on her shoulder, and expects the world (more specifically, us) to provide for her. She does not take responsiblity for her own actions, nor learn from her mistakes, always blaming others. She is irrational. It feels like she is a sulky 16 year old, when, in fact, she is in her late 40s.
I am concerned about her ability to cope with her life, and, especially, to take care of her son. As we are estranged, I don't think she has anyone else to rely on. But I also see that I have a duty of care to my husband. These emotionally gruelling encounters with G are taking a toll on him. He currently 'doesn't want to see her for a while.' I too am angry and feel resentful. But her child is the innocent victim in all of this, and I would like to try to ensure he isn't unduly harmed.
Is G mentally unwell? Are these the symptoms of mental illness or is she just badly behaved? If the former, perhaps we can get her some help. If not, I feel like I am at the end of my resources.