The filmmaker was caught kissing Stewart, the lead star of his Snow White and the Huntsman film, in July and subsequently issued a public statement apologising to his wife and children for "the pain I have caused my family".
Ross was briefly pictured without her wedding ring, but by September the band was back on her finger amid reports the couple was working through its marital problems for the sake of its two kids.
However, the reconciliation has fallen through and Ross launched legal action in Los Angeles County Superior Court to end her marriage this week.
She has hired celebrity favourite Laura Wasser to represent her and has asked for joint custody of their daughter Skyla, five, and son Tennyson, seven, as well as spousal support and attorney's fees.
Sanders has already filed his response to the divorce - he is also requesting joint custody of the kids and is asking to split the legal costs with Ross. He is not asking for spousal support, reports TMZ.com.
In contrast, Stewart's love life seems to be back on track - her Twilight co-star and boyfriend Robert Pattinson appears to have forgiven her for her momentary "indiscretion" with Sanders and they reunited in October.