Both their mothers are from the Angenas (and are cousins). I dont know if their mothers are first cousins or second cousins etc. This would make them second cousins or third cousins etc.
So, not incest even by most modern day standards. Not incest in Renaissance Italy-esque manhwa
My hope is she will have learned from Tia's influence to stand up for herself and wont give the prince the time of day.. I mean, she already spoke up against her father!
1st prince has gotten smart enough to realize that his mother is the sinking ship and absolute obedience to her will be his downfall. Unfortunately, he hasn't yet learned that messing with Florentia is the quickest path to failure.