Is it considered a sin for a wife to remain in a state of suspended animation without divorce after her husband marries another, so that her daughters can see him, and her refusal to return to him unless he divorces his other wife?

Your separation from your husband for fifteen years because he married a second wife is impermissible, for marriage to a second, third, or fourth wife is permissible in Islamic law; indeed, Allah has commanded it. Your refusal to return to the marital home entails the curse of the angels, and seeking divorce for this reason will forbid you the scent of Paradise. Therefore, we advise you to return to your husband for your own good and the good of your daughters, and to avoid Allah's displeasure.