A while back I heard that Sylvia Plath's ancestry had been researched and written up but mostly cut from the final draft of Red Comet (2020), the very fine Plath biography. Another biography declined to discuss Plath's family and childhood to concentrate instead on Plath's "intensity." But where did she get that intensity?
Such questions led me to research Plath's family history (the "stock" or pedigree Sylvia found important when judging her boyfriends), finding patterns and plenty of drama:
Sylvia and Aurelia had African-American cousins. Aurelia's aunt's 1906 marriage to an African-American fits what is now a five-generation family pattern of marriages made to defy a parent or a family.
The first known photo of Otto Plath's parents, Theodor, and Ernestine, Sylvia's grandmother, who died in a mental hospital, I found on a genealogy website. Aurelia Plath, keeper and handler of Plath-Schober-Greenwood-Hughes dirty laundry, kept Ernestine's illness a secret so Sylvia could continue to idealize her dead father, a "pacifist" who to his wife and family was verbally abusive.
Sylvia had no Jewish ancestor. Sylvia had a German father, but she's also Polish, Austrian, and Hungarian. Plath's intelligence and no-nonsense work ethic came to her from both sides of the family. (She wrote: "I don't have time to be intelligent in a fluid, versatile way. I'm too nose-to-the-grindstone.")
For research I used most often two genealogy databases that anyone seeking their own family history can use:
On the free-of-charge database Familysearch.org view billions of pages of digitized info about a billion ancestors, each linked with documentation: census, immigration papers, birth/baptism, marriage, military, and death records, if any exist. That is its advantage over other genealogy sites. Site is run by the Mormon church (also called LDS). Free, but you must register on Familysearch.org as a user.
A good feature: Each ancestor on FamilySearch is assigned a short alphanumeric label so you can be sure, for example, that your ancestor "Anna Campbell" is the same "Anna Campbell" who is listed among 30 different Anna Campbells all born around the same time.
The site Ancestry.com is a gold mine of digitized records. Home subscriptions are costly, but my public library subscribes, and at the library I use it free. Libraries also subscribe to other helpful historical databases such as Newspapers.com. Ask your librarian.
You'll hit walls where there are no records or confusing records. I've learned that guesses are always wrong. These are human records of human beings, so sometimes inaccurate. My stepfather's gravestone says "1916" when he was born in 1919 because when he got his first job in the U.S. there was a mistake when he signed up for Social Security, and he wouldn't correct even the gravestone (ordered years in advance), fearing Social Security would find him out.
My own record says I lived in Ferguson, MO, a place I never even went. I think it's where my landlord had his office. So trust but verify.