Yes I can imagine. I was told by my godmother that I was lucky to have Mum..My mother-in-law had that official diagnosis of NPD before I met my husband, along with anxiety disorder. That's one of the reasons we didn't realise the disorder had developed into dementia, because her behaviour had always been demanding and self absorbed. And to outsiders, as you say, she came over as wonderful. When she died last year, family didn't want a funeral, as my husband and his sister didn't want non family members saying how much she would be missed. My sister in law couldn't bear the thought of a vicar or other facilitator preaching how wonderful she was,when she wasn't. In the end, we had a direct cremation, with no mourners. When we announced to the few family members left this was happening, there was no objection. I don't think any family member cared.
sadly she didn’t want to know how lucky.. at least last year my Dads sister spoke up so at least someone knows the truth!