What is the name of Elrond's twin brother?

Elrond’s twin brother is Elros. The two are the half-elven sons of Eärendil and Elwing, born at the Mouths of the Sirion near the end of the First Age.

After the destruction of the havens, the brothers were captured by Maedhros and Maglor, sons of Fëanor. Rather than killing them, Maglor cared for them. When the half-elven were later given a choice of kindred, Elrond chose the fate of the Elves, while Elros chose the fate of mortal Men.

That decision shaped the history of Middle-earth. Elros became the first king of Númenor under the name Tar-Minyatur, and his descendants included Elendil, Isildur, and eventually Aragorn. Elrond remained an Elf, founded Rivendell, and became the father of Elladan, Elrohir, and Arwen.

Readers sometimes confuse Elros with Elrond’s sons, Elladan and Elrohir, who are also twins. Elros is Elrond’s brother; Elladan and Elrohir are Elrond’s twin sons.