Complex sentence: Gittes slaps her repeatedly until she cries out "She's my sister" and "my daughter!", then tearfully asks if it is "too hard" for him to understand what happened with her father.
Split into simpler sentences: Gittes slaps her repeatedly until she cries out "She's my sister" and "my daughter!". In the midst of this shocking disclosure, Evelyn tearfully asks Gittes if is "too hard" for him to understand what happened with her father.