The Sindar are Eldar -- they were part of the Great March and reached Beleriand.
Quote:
Eldar The Elves of the Great Journey out of the East to Beleriand.
List of Names , The Children of Húrin
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Quote:
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'... and Eldar, the name of the Three Kindreds that sought for the Undying Realm and came there at the beginning of Days (save the Sindar only).' (RotK, Appendix F)
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That's the short (fairly recent) version from Christopher Tolkien. And JRRT on
Eldar according to
The Lord of the Rings.