Yes, SISTERS is a Scrabble word!

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

Is it a valid word?

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

a female person who has the same parents as another person
(Roman Catholic Church) a title given to a nun (and used as a form of address)
a female person who is a fellow member of a sorority or labor union or other group
(slang) sometimes used as a term of address for attractive young women