Basically, if the crown prince kills Carsel, Ines would be a widow. But a widow can't marry a royal (him). So, if the prince can't marry her, she has to become his mistress. But, an unmarried man can't take on a mistress, so, he needs to marry lady Barca first.
tldr he wants to get married so that when he kills Carsel he can take Ines to be his mistress.
The important point is that a married man can take an official mistress. The crown prince isn’t currently married so he cannot take an official mistress but he will be able to once he marries soon. His intention is to make Ines his mistress eventually.
Ooooooh that makes so much sense ong so that's why Luciano got pissedddddd
thanks for the replies !!! <3
im confused by the last part about a widow woman if someone could plz explain