Complex sentence: He confronts Evelyn about her "sister"; after Gittes slaps her, she admits that the woman, Katherine, is her sister "and" her daughter: her father raped her when she was fifteen.
Split into simpler sentences: He confronts Evelyn about her "sister", whom Evelyn now claims is her daughter. After Gittes slaps her, she admits that the woman, Katherine, is her sister "and" her daughter: her father raped her when she was fifteen.