There's a LOT of dispute as to whether or not Caligula had an incestuous relationships with his sisters. A lot of historians believe it to be untruths spread about by Caligula's dissenters. It's no secret that Caligula was nuttier than a bag of squirrel poop. While he was much loved when he became emperor, he took a very dark turn in his reign and it's believed that it had to do with illness, either Epilepsy or Syphilis. He became tyrannical, cruel and outright crazy which led to his assassination.<quoted text>
I may be wrong about Caligula being married to his sister but he did have sex with her and treated her as his wife in fact leaving her his heir.
It's also true that Drusilla was his favorite sister. She was the only one he trusted to the point where he had his cousin Germanicus, who at point was deemed his heir, put to death because he believed Germanicus was plotting against him. Drusilla was no longer considered heir once his daughter, Julia Drusilla, was born and Drusilla ended up dying of fever.
There's no dispute about that. However, you mentioned Caligula and his sister being married and I just wanted to tell you that was incorrect.<quoted text>
Incestuous marriages were common though in ancient history.