Is it forbidden for a wife to refuse her husband's marriage to a non-Muslim woman? And is it forbidden for a wife to hack the electronic account of the prospective wife to ascertain her intentions?

A wife is not permitted to prevent her husband from marrying another woman. Her displeasure should not lead her to spy on the woman by hacking into her account, as spying is forbidden, even on a peaceful non-Muslim, unless there is something suspicious emanating from them. It must be noted that a Muslim man's marriage to a non-Muslim woman is not permissible unless she is a chaste woman of the People of the Book. Furthermore, if a man marries another woman, he must treat both wives justly.