See, there's theories that I love that probably wouldn't make it to the film.
book theory:
Aegon never had kids. Rhaenys had a bastard with a Minstrel which explains Aeny's sickly nature and Visenya had Maegor via dark magic which is why he never had kids, but monstrosities. He should have had for more than 2 kids if he was fertile. All Targaryen primogeniture is based on one huge lie.