Yes, BIKINIS is a Scrabble word!

Letter values in Scrabble US: B 3, I 1, K 5, I 1, N 1, I 1, S 1.

Is it a valid word?

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

an atoll in the Marshall Islands; formerly used by the United States as a site for testing nuclear weapons
a woman's very brief bathing suit

Also: two-piece.