"Don't worry about that, Nephew. Your Daddy has been fucking me since long before he even met your Mom. And your Mom not only knows about it, she loves to watch. She'll even beg to lick your Dad's cum out of my pussy. Maybe I'll call her in when we're done so she can lick all of your cum out of me as well."
"Since Mom doesn't fuck him as much anymore, Dad's cum qualifies as "something old", "something borrowed", and "something blue (balls)". The "something new"? That's the baby Dad put inside us when he fucked us!"