The answers to Were ANY of the Elves in Tolkien twins? include Elladan and Elrohir as twins. Is there any cannon sources backing this up? Wikia is a dreadful source for Tolkien canon, is it stated anywhere that the sons of Elrond are twin brothers?
130 Birth of Elladan and Elrohir, sons of Elrond.
Return of the King Appendix B "The Tale of Years" (ii) The Third Age
Of course this doesn't preclude the possibility that one was born early in the year and one was born late, as sometimes happens with human children, but writings in Morgoth's Ring debunks this notion; the Elvish gestational period is twelve months:
A year passed between the begetting and the birth of an elf-child, so that the days of both are the same, or nearly so.
History of Middle-earth X Morgoth's Ring Part 3: "The Later Quenta Silmarillion" Chapter 2: "The Second Phase" Laws and Customs Among the Eldar