I can't live like this anymore. What should I do?

I have a wife and 2 beautiful children. Before we had met I was in a relationship with her sister. Her sister tragically ped away and now I am in a relationship with her. We met at her sister's funeral. I never brought up my relationship with her. Now we are angry with each other due to insignificant arguments. She is starting to get in touch with her mother more, and I don't know if her sister ever told her mother about us, and I don't want it to happen. Should I confess now, or should I wait and see what happens?