How can I talk to my mother about my father cheating on her?

If it's something that you really have to do, find out if your mother is experiencing any doubt herself. Ask her if she's noticed your father acting differently or spending less time at home. If she flat out refuses any changes, she may be in denial, otherwise it may allow you to have a healthy conversation about the issue. If you feel as though it's only going to get worse, then you might be forced to confront your mother with the truth.