The word stepsister is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 13
Highest Scoring Words With Friends™ Play In The Letters
Seven Letter Word Alert: (27 words)
persist, pesters, petites, pettier, presets, presses, pretest, priests, resists, respite, resteps, retests, septets, sestets, setters, sisters, sitters, spriest, sprites, streets, stripes, tersest, testers, testier, tipster, tresses, tsetses