While
Maeglin was especially icky, betraying the
"jewel in the crown" elf city to rule over the remnants
of Gondolin with a forced marriage a la
Ar-Pharazon/Miriel,
first cousin's marriage is not only fairly common with
royalty (and in Appalachia?
) but I don't think it is
technically incest.
And he is a free agent throughout while
tWKoA
is more then not a tool (albeit apparently a willing one,
and in Angmar pretty much a local free agent), so
+ + Maeglin