Family Secrets in Muddling Through As Best I Can

Okay, this is different from my usual posts, but here goes.
Years ago and before his death but after Mother’s, my father revealed some secrets about my mother’s family.
One of my uncles, Ray, and his wife Naomi had an adopted daughter named Nina, for years older than me.
Dad told me before his death that Nina is not actually adopted, but is the product of an affair between Aunt Naomi and one of Uncle Ray’s and my mother’s cousins . my Grandmother confirmed this when I asked her albeit reluctantly.
Nina sent out a message that she wants to locate her birth parents.
I believe I may be the only living person who knows the truth, including the name of the cousin. He’s in his 80’s and the only person still living in the triangle.
I’m not sure what to do, or if I should keep silent and do nothing. Nina has apparently wondered for years, but I also don’t want to be the one to destroy her image of her mother.
Give me your opinions please. I’ve seldom faced such a moral quandary, where I’m afraid the truth is more damaging than the alternative.