I agree with Milena’s last sentence. In my opinion Marcella’s relation to Cal is like a rush. You have to think about what she has done in the last few years. First her husband is murdered in her own house, then she has to live in a house with her parents in law she doesn’t like and who always remember her of Robert. And in this whole time she doesn’t even have a friend to talk to. Her only gleam of hope is her daughter. In this situation appears a young man, who lets her feel alive and young again. This rush lets her forget about all her problems.
If Cal would tell her now, that he more or less directly helped to kill her husband, this illusion of love and a future with him would totally break. In my opinion she would react with big anger. She would accuse him, that he utilizes her weakness, and forbid him to approach her family ever again. Then she would call the police because she feels humiliated by Cal.