I know it's not easy and you are right in the thick of it. And you're in the midst of it even if you never wanted to be. If he was banging some woman from the office, you probably wouldn't know. But this being your neighbor and a close relative drags you into their drama.
Don't get dragged into their drama. Claw out of it instead. And if they try to pull you back in, for God's sake don't go willingly.
If your mother asks you, don't lie. But do not volunteer this information. Why? Because inevitably this will turn into a "shoot the messenger" situation.
If you want to tell your father to cut it out and that you know, well, I can't stop you. But I doubt it would help things much. He is engaging in exceptionally reckless behavior. He would have to be a fool to think it wouldn't be noticed by anyone.
And if you want to tell your aunt to cut it out and that you know (it takes two to tango, yes?), then I can't stop you from doing that, either. I suspect the best candidate for a confrontation, if you are hellbent on having on, is with her. But allow me to repeat my first sentence. Hard as it is to do -