Was I wrong not to say anything?

In the last 4 months my Mum has found out that her husband of 30 yrs and my Dad has been having an affair for 3-4 yrs she has decided to stay with him at this stage. 2 of my brothersand I suspected something was going on a yr or so ago and assumed that she knew, and one of my brothers confronted him he denied anything was happening and we believed him. Mum is very upset with us that we didnt say anything at the time about our suspicions. I feel horrible about the whole situation but as a 27yr old living away from home was I wrong. to leave my parents relationship to them.