Celebrian Who?

[Legolas]

Fame by association doesn't always bring an important character's spouse/family members into the main stories (being the stories of the First Age and the War of the Ring).

Elmo, brother of Telerin lord Olwe and King Thingol; Nimloth, Dior's wife and Elwing's mother; Elenwë, Turgon's wife; Anaire, Fingolfin's wife; Fingon's wife/Gil-galad's mother... No matter how big a part their relatives had in the story, these characters are given little or no character at all.

Celebrian's case (in being left out of The Lord of the Rings plot) is due to the time in which she lived - Elrond was born in the last century of the First Age and Celebrian in the Second Age sometime. This, of course, falls in the gap between the War of Wrath and the happenings documented in The Hobbit, the gap of knowledge left by reading only The Silmarillion, The Hobbit, and the main body of The Lord of the Rings. Her fate does advance the story of her family though - Elrond is separated from his wife and certainly wished to see her once again, Arwen is prematurely separated from her (i.e. long before she would've expected should she remain and wed Aragorn), and Elladan/Elrohir gain the deeper hatred for orcs that led to their companionship with Aragorn and their accompanying him on the Paths of the Dead.