The Prince's Wife. Tales of Tavamara, #5.5

Aradishir has always been the loner amongst his siblings. That he's finally taken a full harem astonishes the whole court. He will someday have to take a wife as well, but that's a distant concern, especially as he's the youngest. Then he meets Princess Relanya: smart, funny, as interested in helping people as he is, beautiful, and she seems to fit in perfectly in Tavamara, despite having never visited before.
She is the woman of his dreams, and he would do anything, everything, asked of him to earn the honor of taking her as his wife. She's also betrothed to his brother, and no woman on earth, especially an older and wiser woman like Relanya, would ever choose the little brother when she already has the crown prince.