"
Arwen was not an
elf, but one of the
half-elven who abandoned her
elvish rights."
Letter 345
If this is true, then why does
Tolkien include her marriage to
Aragorn as one between the Elder and the Adani?
Letter 345 was the last letter JRRT wrote in his life - the last one to be included in the original edition of "Letters", at least. A cattle breeder had asked for permission to use names from LotR, which JRRT was not happy with, offering to invent some Elvish-sounding names instead.
My guess is this could be an inconsistency due to the fact that JRRT never "finalized" his 'Silmarillion' tradition.
Strictly speaking,
LĂşthien was
also half-Elven. And the "only" other "precisely"
half-Elven was
Eärendil. Dior, Elwing,
Elros,
Elrond, Elladan, Elrohir and Arwen were more mixed; Dior (statistically speaking) a quarter-Elf, the rest more than half.
My readings, and more so discussions here on TTF - with posts peppered with quotes of JRRT's writings from diverse sources - give me the impression that it was the admixture of human to Elven that gave the
Valar headaches. Or rather, that Dior, Eärendil, Elwing, Elros and Elrond were considered mortal until Eärendil and Elwing managed to sail to
Valinor. Only then, by dispensation from
Eru, was
Manwë able to give first Eärendil and Elwing, then Elros and Elrond, the choice of which of the two branches of the Eruhini they wanted to belong to. Eärendil and Elwing never set foot on
Middle-earth again, which Elros and Elrond had never left up to that time. (JRRT never mentions whether Dior ended up in the
Halls of Mandos.)
And the the asymmetry between the choices of Elros and Elrond. The former's decision was binding on all of his descendants. while the latter were bound to Elrond's remaining in Middle-earth. While he did so, they certainly lived the life of
Elves, Elladan and Elrohir for 2,891 years until Elrond's departure, while Arwen was 111 (!!!) years younger than her brothers. Arwen then spent the next 120 years as Aragorn's queen - also not exactly a normal human lifespan, but then neither was Aragorn's.
A last thought: statistically Elrond was
less Elven than his children. But JRRT never counted the union of Elrond and
CelebrĂan among those between the two Kindreds ...