Sibling rivalry, adultery and dungeons are coming to Threshold Repertory Theatre in April as it presents “The Lion in Winter.” “The Lion in Winter” by James Goldman is a modern-day classic often described as a medieval “Who’s Afraid of Virginia Woolf?” Comedic in tone and dramatic in action, the play tells the story of the Plantagenet family, which is locked in a free-for-all of competing ambitions to inherit a kingdom. Queen Eleanor of Aquitaine - also the wealthiest woman in…