Hmm.
It can't be to do with their children: Turin and Nienor never had any. It can't be their parents, because
Finwe never had any (named). It can't be anything to do with their deaths, because Idril never did.
Is it something to do with them all having two suitors (of varying degrees)? Aragorn has Arwen and Eowyn, Luthien had Daeron and Beren, Idril had Tuor and some elf who just doesn't get mentioned enough for me to remember his name (
), Niniel had Turin and Brandir, Turin... well, he was one of
Finduilas' suitors, but also I guess Finduilas & Niniel. And Finwe had Miriel and Indis.
But those last two kind of break it: while the other four had one they rejected and one they married, Finwe was married to both of them, and Turin didn't know both at the same time.
If it
is this, is your caveat Tar-Miriel? (Or Tar-Ancalime, who was a total fairytale princess when it comes to rejecting suitors.)
hS