The princess definitely overreacted, but from her point of view, Julia is the "other woman" who caused a huge blow to her honour. And the princess obviously feels like any such blunder will make it harder for her to catch up to the first prince in terms of influence.
Also, wow, Vasily, what a bold move. Could've picked a better way/time/place to do it, but let's say you were unable to think it through because of all the stress.
So far most characters behave like humans driven by very understandable emotions, and that's genuinely lovable.