http://www.suttonclonard.com/
What if the theory that Isabella had an affair with the Earl of Clonard is true.
Well, if it is true, then it means that human history hitherto has been the consequence of the affair between Isabella and the Earl of Clonard.
If it isn't true, then it means that human history hitherto has been the consequence of Isabella and the Earl of Clonard not having an affair...
It's a bit hard to chose as a point of divergence an event that we actually don't know whether or not it actually happened.
But I guess that you can make the affair develop in way that we can be certain that such an affair couldn't have developed in (had it happened, which we don't actually know) and let
that be the point of divergence.