As far as I know there are three events that give you a daugther as hair, I think one is just a event that gives a female heir with average strength because you dont have a male heir in forever, one is if your king dies and your ruling dynasty would change gives you an opportunity to get the daugther of your previous king as ruler with average stats and the last one is called something with ambitious daugther which gives you a pretty good female ruler.Regarding the pop up that gives you a female heir (not sure on the term for it) - you get a 'surprise a daughter !' and can either recognize her as your heir or not:
Is that designed to give you an unusually powerful heir ?
I ask, as in a Byzantine game with the usual 2/2/2 idiots, I got offered a 6/6/6 female heir ! She didn't even have a hunting accident !
Thinking about it, I've seen 10+ pt heirs with other nations, very different to the male heirs I'd otherwise get, hence the question.
Those are the only events I know of that give you a choice to get a daughter as heir or no heir at all, however there are a few other events that may result in a above average female heir (its called starlight and is basically the female version of lux stella which lets you "name" your heir after someone and choose from 3+ choices of above average rulers)
TLDR:
It´s probably one of the events designed to give a powerful heir.
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