@PlainSpeakingStraightTalking
NPD is rare but this is most likely to be because the sufferer doesn’t believe that there is anything wrong with them. Everything is someone else’s fault and they are perfect but very unlucky because people are always awful to them. In which case, you will NEVER get someone with NPD going to a therapist or GP about their MH therefore they are never or very rarely diagnosed. It’s a vicious circle.
It is diagnosed by proxy, by the victim’s therapist, but usually it’s just the victim who’s diagnosed with c-ptsd/anxiety/depression and so on.
The other issue is that narcissism is a spectrum and many people have N Traits such as over-entitlement, lack of respect for their adult children, trampling on boundaries and general Cheeky Fuckery. Making nasty little digs and deniable insults is toxic whether the person is diagnosed or not.
The OP hasn’t said her MIL has full blown NPD, has she? What she says is that the MIL is narcissistic and passive aggressive and you don’t have to be a Psychologist to pick that up.
FWIW my mother has been diagnosed by proxy as a disordered malignant narcissist, a sociopath and an abuser. The only people who know about this diagnosis are my DH, DB and DSIL. She was diagnosed by my two separate therapists after almost 3 years of counselling. I haven’t made it public though and there will be nothing on HER records which probably explains why it’s so rare.
That doesn’t mean toxic behaviour somewhere on the narc scale is rare though, and those of us who have experienced it can spot it a mile off.