Complex sentence: Marcia takes advantage of the situation to marry a man she doesn't love and get pregnant, but after an abortion, she uses her wits to seduce Ignacio and steal Mariana's son, whom she passes as hers to make Ignacio marry her.
Split into simpler sentences: Marcia takes advantage of the situation to marry a man she does not love. She becomes pregnant but after an abortion she seduces Ignacio and steals Mariana's son, whom she passes as her own to make Ignacio marry her.