[2023] having multiple wives

Lord0ver said: first choice - attempt to win my heart. She is already my wife

Marriage in the game is what it was back then: a "contract" between two noble families. So no, being married to someone doesn't mean you're already in love with them.
If she succeeds in the scheme, she'll become your lover - you'll have an opinion boost with her and a higher chance of bearing children. And she'll not becone your primary wife, no.