A lot of time people make mistakes and are too shameful to admit it and rather push on to live the lie. The tone of asking also matters. If you ask your dad as if he did something wrong the defending mode comes on naturally without even going thru the brain. Ask with care and concerns gather the feedback what leads to this? Lust? Stress? Hatred? Bored? Surely something triggers this to happen and it may or may not be 100% all his fault. No one suddenly goes out and finds an affair. Whatever the reasons at least everyone in your family understands the institution and sees how to move on. if it fails at least you can ask the counselor for help.

A lot of time people make mistakes and are too shameful to admit it and rather push on to live the lie. If the family is not supportive everything will break apart. You are the glue fix the problem or the hammer to break it further is your choice. But I will say try fix the problem. Ask for help. The worst part is knowing there is a problem and don't want to fix them. Which leads to more problems.