I’m not sure if this is the best place to ask this question, but since it is about how we record history, historians seem like the right people to ask.
My grandmother’s parents were artists. Most people won’t know their names, but would recognize one of my great-grandfather’s works. They do both have Wikipedia pages. My local university has a small exhibit about my great-grandmother. There have been a handful of books written about them. Someone recently made a documentary about my great-grandmother that I doubt anyone watched. Basically, there’s a little bit of academic interest in them, but not a ton.
My great-grandfather was a known philanderer. Biographies usually touch on this and his numerous famous affair partners and subsequent wives. This is important for my family secret.
My grandmother recently revealed to my cousin that the woman everyone assumes to be her birth mother is not actually her birth mother. Apparently great-grandmother couldn’t have children. When they were living in London, great-grandfather had an affair with the live-in maid. The maid became pregnant and great-grandmother saw her chance to be a mother. Great-grandfather left for a job in New York City (unclear if before or after they found out the maid was pregnant). About a year later, great-grandmother showed up in NYC with a baby and her own name on the birth certificate as the mother. I have no idea how consenting the maid was to any of this or anything about the maid’s identity.
My question basically is 1) is this something important for art historians who study my great-grandparents to know and 2) how do I share this information with historians? I certainly don’t want to do anything until after my grandmother passes on (she’s deeply embarrassed about the whole thing), but it also doesn’t seem like something that should die with her.
The best thing you could do would be to compile and preserve any records you have relating to this story and to the family in general. You could possibly arrange for an oral historian to interview your grandmother and dig into this story, or you could even record an informal interview yourself. You should give these records to the local university (or another institution that already holds papers related to your great-grandparents) with the requirement that any documents that are potentially embarrassing to your grandmother are sealed for X number of years. This way the information is preserved in perpetuity while avoiding family conflict.
Just because you may feel that this story is not significant now doesn't mean that a future biographer or art historian wouldn't find it so. Scholars are always looking at things through new lenses. There is increasing interest in examining the work of artist couples together rather than in separate monographs. Since both your great-grandfather and great-grandmother were artists, there is reason to believe they influenced on each other's work.
What may seem like salacious gossip could potentially be the key to a greater understanding of their artwork. The inability to have children can be deeply traumatic to both partners in a relationship, and there's every reason to consider that this would have an impact on their work. You can look at the work of Frida Kahlo as an example of this.
While your great-grandparents may not be household names likes Kahlo and Rivera, you do mention that their work is recognizable and biographies have already been written. It's impossible to predict how future scholars will deal with this story, so preserving any evidence for them should be your first step.