[Legolas]

Legolas is Sindarin by blood, and Silvan by culture.

As noted above, his father, Thrandiul, and a number of other Sindarin elves migrated south to Middle-earth from Beleriand at the end of the First Age.

East of the Misty Mountains, a 'melting pot' of elves had settled and mixed together. There were assorted elves that wandered away from the journey, including some of the Nandor that didn't accompany Denethor on his trip to Doriath to rejoin the Sindar. The Green-elves had also mixed in some, along with Avari that had slowly migrated west possibly. These different backgrounds mixed together to form a blend called Silvan. While not Silvan by blood like most of his Mirkwood community, Legolas was still a part of that culture.

There's an appendix in Unfinished Tales on the 'Sindarin princes' among the Silvan lands east of the Misty Mountains.

Also, 'Sindarin' is not a race. 'Elven' is the race. The Sindar are a kindred.