A couple high-ranking goddesses like Diana and Callisto obviously never heard the maxim 'sisters before misters.'

Callisto got tricked into some freaky business with Jupiter, king of the heavens, and everyone started hating. She was a nymph in the service of Diana, goddess of the moon and an avowed virgin. Since Diana wasn't getting any, ain't no one in that retinue getting any. But Jupiter never let anything like vows, or marriages, or people saying no stop him from doing what he wanted to their bodies. And Callisto was no exception.

He shows up in the form of none other than Diana and makes an indecent proposal when Callisto's all by her lonesome. Callisto's preggers some time later, and everyone figures out what happened. Diana lost her damn mind and turned Callisto into a bear. Despite the transition, Callisto gives birth to a human who grows up into the ancient Greek version of an overzealous park ranger. 

Afterward, Juno (Jupiter's unsympathetic wife) has it out for Callisto. She sets it up so that Callisto's son sees his ursine mother and goes for the kill with a spear. Jupiter feels a little guilty and turns the pair into star constellations...we call them the big and little dippers. Or, in most other countries, the big bear and little bear. It's a story that likely goes back thousands of years to the very beginning of myth. Reading it makes you a part of one of the most ancient traditions our species has to offer. Don't you feel special now?

Rubens was inspired by another painter's version of the story. But there's a huge difference. Most other painters focused on how pissed Diana was that Callisto was tricked into breaking her oath. Rubens makes Diana look super horny over Callisto's pregnancy glow. This situation takes an interesting turn when considering that Callisto is modeled off of Rubens' wife at the time. Mayhaps the artist was making an indecent proposal of his own.