In its earliest days, a broad racketeering-conspiracy and sex-trafficking case has zoomed in on a single relationship.

The jury of eight men and four women in the federal sex trafficking trial represent a diverse swath of the New York City area from various professions.

Lawyers for Sean “Diddy” Combs say they will question R&B singer Cassie on Thursday about how infidelity fueled the combustible, nearly 11-year relationship between the two, as cross-examination of ...