What is Peter-in-law?
1.
A relational distinction. If you sleep with a girl, then your buddy sleeps with her. You are his peter-in-law. The relationship is one way. You would not be his peter-in-law unless you sleep with that girl again *after* him.
Sean Penn (Madonna's ex-husband) is the peter-in-law of Guy Ritchie (and let's face facts, a bunch of other people too!)