When asked by Michelle why she never speaks, she simply doesn't answer.
On her blog she says that her parents had her see a counselor for withholding her speech.
Josh's idea of revenge is sick, twisted, and dark like nothing else in the series up to this point, and is handled excellently by the writers, who succeed in making it feel dark and actually sort of spooky and disturbing.
There is implication however, in the first episode and others, that she does speak to her parents only occasionally, although after Tony's accident she starts speaking again.
Expressing her feelings of dislike to Tony, he snidely remarks to her that she "doesn't fool him".
Hats off to director Adam Smith who builds a spooky and foreboding atmosphere without which this episode might have fallen flat on its face, and kudos to Kaya Scodelario, the youngest of the frequently seen Skins actors, but also perhaps the most talented and promising.
She perfectly embodies the mysterious, intriguing, and dangerously sexual Effy.
Contrary to what some say, "Skins" IS realistic in some ways: the characters are believable and even some of the more outlandish events in the series are unsettlingly authentic, such as most of the partying shown.