My dad

I wouldn't say anything

No, it will be a $hit storm and you dont need that.

Need not report to your mom. Your dad is getting what your mom failed to give him. Having said that any consensual sexual relationship between adults is quite ok and let such a relationship within the privacy of your home flourish. May be someday, your mom would come to know about it and might enter into a fruitful 3-some arrangement among your mom, aunt and dad.