There is not much you can do, other than bringing stress and unhappiness in the family. What would be your father's re-action if you told him, about your mother' s dalliance? Would he be willing to listen to you or will he take sides? You are also complaining about his brother.
Your speaking to your mother is not going to help. She will not pay nay attention to you and on the contrary you will distance yourself from her by your action. She will rightfully say that you have been snooping on her and impiging on her privacy.
You have your mother's welfare at heart, but remember that this affair is going on with her consent. She knows what she wants and while you may think it is not good for her health and heart (love), she may not agree. Since the affair is going on for such a long time, the possibility of it ending on your intervention seems remote.
Get some elder who your father respects to have a word with him, if you are so stressed. My advice though is to turn a blind eye as long as you are not directly affected.