(Spoilers extended) Looking through the list of Targaryen kings, I get the sense the stereotype that emerged that they were all either great or insane is a myth.

Aerys ll and Aegon IV seem to be the only ones who were actually terrible. And even then the latter wasn’t really insane, he was just an asshole who was bad at being king. The closest you get besides them is Maegor who was an asshole but he was very competent.

And only like Aegon I Jaeherys I and arguably Baelor were truly great

Most Targaryens seem to be just ok. Viserys I and II, ok to good. Maekar was ok. Aerys I was ok. Aegon III was ok, I don’t think the dragons dying out can really be blamed on him.

Not to mention how most of them didn’t actually marry their sisters, they still married their extended cousins or family but so did every other lord in Westeros.