Yes, SIBILANT is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, B 3, I 1, L 1, A 1, N 1, T 1.

Is it a valid word?

SIBILANT is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

of speech sounds produced by forcing air through a constricted passage (as `f', `s', `z', or `th' in both `thin' and `then')

Also: sibilant consonant.