Is sitter a word in the Scrabble Dictionary

Yes, sitter is playable

...and is worth 6 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Sitter" Definition

noun

A brooding hen.
A person who cares for young children when the parents are not home; a babysitter.
A person who poses or models, as for a portrait.
One that sits, especially:.
one that sits as.

biographical name

Willem de 1872–1934 Du. astron.