James II's daughter, Isabella (1676-1681) died a few months before her fifth birthday. She was up until that point the longest lived of Mary of Modena's children. And her death reportedly affected Mary very deeply.
What might the effects have been with a surviving Lady Isabella Stuart? If Isabella had lived, would Charles II have insisted that she be brought up Anglican or allowed her parents a say in the matter? Would she marry Protestant or Catholic (maybe the future Regent of France)? And should the Glorious Revolution still occur, does she go to France in exile with mom and dad, or does she remain in England, at William and Mary's pleasure?