Not quite... here's the full nasty family tree...
Elwing was the daughter of Dior and Nimloth. Dior, whom you may or may not recognize as the son of Beren and Luthien, was clearly half-elven, as Beren and Luthien were human and elf, respectively. Nimloth was (as far as we can tell) a full-blooded Elf.
That makes Dior half-elven and Nimloth full-elven... so Elwing would be 3/4 elven.
If Elwing is 3/4 elven, and Earendil is 1/2 elven, that makes Elros and Elrond 5/8 elven and 3/8 human.
Elros chose to become human and was the progenitor of the kings of Numenor (if memory serves). This is why the Numenoreans are so long-lived. Elrond, of course, chose to become elvenkind, and was granted immortality.
This makes Arwen 13/16 elf, for those keeping score at home.
--The Sigil