The Bride Was His Sister
yes, yes, both triteness AND plausibility troubles, but I was reading a story where the best man takes the bride and thought "this could only be more perverse if she turned out to be his sister". which is strange because bro-sis sex not my fetish.
How about this:
Kyle shares an apartment with Floyd, whose fiancee is doing a tear abroad. Because of the time difference, Floyd & his gal always skype after Kyle is asleep.
At the end of the semester, Floyd invites his roomie to be the best man at the wedding. They arrive at the resort where the ceremony is planned. when the bride to be enters for the rehearsal, Kyle realizes that she is his half-sister, that gal that his Dad sired with his secretary....