Yes, SITTER is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, T 1, T 1, E 1, R 1.

Is it a valid word?

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

Dutch astronomer who calculated the size of the universe and suggested that it is expanding (1872-1934)
an organism (person or animal) that sits
a person engaged to care for children when the parents are not home
a person who poses for a painter or sculptor
a domestic hen ready to brood

Also: willem de sitter, baby-sitter, artist's model, brood hen, broody hen, setting hen.

Not playable in Scrabble US: BABYSITTER.