Question for hardcore LOTR readers......

[Guest Lord of The Curry]

Ok, I'm just browsing through FOTR the other day and I noticed a quote in The Council Of Elrond chapter that caught my eye I never saw before. It described the two sons of Elrond, Elladan and Elrohir and how they often rode with the rangers of the North, "never forgetting their mothers torment in the dens of the orcs."

 

What happened to their mother? Who is their mother? Did I miss something in the LOTR series or is this something in Unfinished Tales or The Simirillion?