I know the truth and what my relationship with him has done to me. I went to the therapist and got complete validation for my suspicions. Infact, he wanted to give me a book on n behavior and decided I didn't need it. I have already done 10 years of journaling, which the therapist told me was therapy. Once I received the validation of my husband being an n, I feel satisfied and, in a way, exhonerated with myself and my self-esteem.
Some people are like that.
If she's advanced in years, she will probably want to leave things as they are. You will be a great support system for her if she ever decides to ask questions.