“Probably the biggest influence on the early Beatles. “
Biggest is debatable.
But McCartney name-checked them in that Let ‘em In song - along with Martin Luther.
Why debatable, make your case!
In fact when it comes to the vocal harmonies, there is no question about it, as the Everlys were bigger in Britain than they were in the US, and there was a duo in the UK at the time of the early Beatles that sounded just like the Everlys, called the Allisons (there was also another one whose name I forget at the moment.) That’s my case.
Buddy Holly was a big influence on the Beatles, too. In fact, I can hear some Holly in the Everly Brothers. And some Everly Brothers in Holly.