i wrote a story along those lines once, i might be able to help, mine had the aunt catching him on the way out of the shower or out of the bed, very cliche, but it works in a pinch. would you have the aunt be willing?
Jon
The old, coming home suddenly - power outage, storm, fire, flood, pistol packing kids in school, works good for me.
Just think up a good plausable reason for the young man to come home early and catch his Aunt - masturbating while watching TV porn, taking a bath, nude in the swimming pool, with another woman making love - all will work! Let you imagination go!