Hi Mark,

This is an awful secret to be holding onto. I can’t tell you what to do as you know your mum better than I do. The question I would ask myself is would I want someone to tell me I they knew I was being cheated on? If you ask yourself this and the answer is yes and you think your mum would feel the same, then talk to her. Especially if she is helping your aunt out right now- she might feel angry that she let her carry on doing that in the circumstances. If your father is never going to tell her then it might be down to you to let her down gently. Perhaps think through how you will tell her beforehand rather than just blurting it out. It will need to be handled with the greatest of care. Expect a whole range of emotions- anger, sadness, confusion- so try to be patient.  

If you do decide to tell her, then you are going to have to be a big support for her to help her cope with the news and the aftermath whatever that might be. It sounds like perhaps this relationship is more emotional than physical, as your father is probably already aware of how it will affect his marriage but has continued to pursue it.

Ultimately, this is not your news to deliver, however things like this usually come out somewhere down the line- so it’s a choice of who you want your mum to hear it from.