So wait... if the Autumn Queen is Noam's sister, then does that mean that Theron is the cousin of the Autumn Princess?

Yep! Theron has quite the family tree:

Noam’s younger sister, Nikoletta, married the Autumnian king, Caspar, and they have a daughter, Shazi, who is the future Autumnian queen and also Theron’s cousin.

Noam’s wife, Melinda, became a Ventrallan princess when her sister, Brigitte, married the last Ventrallan king (so Melinda isn’t a Ventrallan princess by blood, but by marriage). Brigitte and the last Ventrallan king had a son, Jesse, who inherited the crown when his father died. Jesse is Melinda’s nephew and also Theron’s cousin. 

So Theron is related to two different kingdoms’ rulers. 

Wouldn’t be a real monarchy without weird extended relatives.