I can't remember the title of a film, it's a period drama probably made in the 70's or early 80's. It's about a girl whose father before he died lost her in a card game. She moves in with the mother and son, and uncovers that the husband didn't really die but the family faked it for money. She ends up falling in love with the son and they get married, not before the mother tries to kill her.

It's "A Hazard of Hearts" from 1987, starring Helena Bonham Carter.