[Cecily]

William’s mother Diana supposedly is descended from James II via an illegitimate line of ancestors.

[Campion]

She's descended from Charles II, not James II, through an illegitimate line, and there's no doubt about it. This fellow is her distant ancestor.

[ChronicMA]

William’s mother Diana supposedly is descended from James II via an illegitimate line of ancestors.

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Diana is descended from Charles II, the king who was restored to the throne and was something of a party guy with lots of children but not one legitimate kid who could inherit the throne. So the throne went to his younger brother James II instead. James was father to Mary and Anne who both became queen regnant but neither had surviving children.