Yes, SISAL is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, S 1, A 1, L 1.

Is it a valid word?

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

a plant fiber used for making rope
Mexican or West Indian plant with large fleshy leaves yielding a stiff fiber used in e.g. rope

Also: sisal hemp, agave sisalana.