Yes, SIDEWINDER is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, D 2, E 1, W 4, I 1, N 1, D 2, E 1, R 1.

Is it a valid word?

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

small pale-colored desert rattlesnake of southwestern United States; body moves in an s-shaped curve
air-to-air missile with infrared homing device

Also: horned rattlesnake, crotalus cerastes.