This Was a Woman (1948)

Sylvia Russell is power-mad and has some set goals.

In order to achieve those goals she is sadistically devoted to her son and daughter, able to possess and direct them: she subtly ruins her daughter's marriage by corrupting a servant girl whom she then uses to compromise her son-in-law; she slowly breaks her bumbling, unambitious husband's confidence; and she finds in her husband's executive friend the man she desires.