Elladan and Ellrohir are described as twins. The word "twin" might not have been used, but they literally were "described" as twins.

Lord of the Rings > The Return of the King > Book Five > Chapter II The Passing of the Grey Company:

> "...and two tall men, neither young nor old. So much alike were they, the sons of Elrond, that few could tell them apart:..."

Also, Lord of the Rings > The Return of the King > Appendix B > The Third Age:

> Year 130 Birth of Elladan and Elrohir, sons of Elrond.

Pretty damn explicit. If Tolkien didn't want them to be twins he would have given them different birth years and not explicitly described them as being hard to tell apart. Soooo I think it's super safe to assume that Elladan and Elrohir who were born in the same year, who looked super alike and were never apart were twins.