Harwin was married to your little sister Rhaenyra now. You watched him court Rhaenyra, after years of courting you, as if he had suddenly changed his sights when he heard about your engagement. At first you thought Harwin really liked Rhaenyra, and Rhaenyra thought Harwin really liked her
But before long, surprisingly to you and heartbreakingly to Rhaenyra, Harwin's true purpose was revealed. Harwin's marriage to Rhaenyra was far from a happy one. They fought a lot, Rhaenyra accusing Harwin of all kinds of things because her bed was empty at nights.
And Harwin simply didn't care about Rhaenyra or her feelings. The person Harwin was trying to impress was you again after his marriage to Rhaenyra. He was obviously dishonorable enough to do this and didn't even have the dignity to admit it to himself, but he kind of used Rhaenyra as a stepping stone to get closer to you. Since he was now your brother-in-law, his closeness to you didn't even attract attention.
Both Harwin and you were married to different people, Harwin didn't care about his own marriage and would prefer you not to either. You, your husband, your sister Rhaenyra and Harwin lived together in dragonstone. With its small entourage and its distance from Kingslanding, it was a place where sins could be committed easily.
You and your husband were the first of this strange love quartet to get married, and right after you got married, you settled in your castle, Dragonstone, to rule it. When Harwin and Rhaenyra married shortly after you, you were the one who invited them to live with you in Dragonstone, which you now see was such a stupidity. Although neither you nor Rhaenyra knew it at the time, you were the one who gave Harwin this space
Harwin had come to you today, as he often did, while you were wandering around the dragonstone gardens. He grinned slightly when the conversation between you two turned to his marriage. His voice was as careless as it shouldn't be
"I assure you my wife doesn't care about me anymore, princess."