@AugustusR Cousins marrying cousins is legal in many countries, even in the US it's still legal in some states. These days with our knowledge of the genetic consequences of consanguinity it has become a big social taboo in most countries, but that's a phenomenon that is less than a century old. Before it was actually considered a good thing, it meant any inheritance or business stayed within the family, there was built in trust between all parties, in times where people didn't live as long you didn't have to have a long courtship, you already knew everything about the other person... I'm not sure how often it was actually done between first cousins, but it was definitely a good way to strengthen family bonds.
Leylin coming from the modern world and therefore having that knowledge is listed several times as the main reason she never even thought of Stand as a potential match, and why her father's announcement took her completely by surprise while everyone else expected it. She probably dismissed any hints of it out of hand. It's also why her refusal was so blunt. If her arranged marriage had been with someone else, she might have considered it, and then found a less brutal way to cancel it.
For Stand he was told immediately that marriage was the plan, and then he thought he was so lucky because he genuinely liked her. As a result he never made much of an effort to get her to love him the same. He probably just thought that even if she didn't love love him, since she definitely liked him a lot, it would be fine.
I feel sorry for him, but at the same time if he'd made an actual effort at making her fall for him she would have rejected him sooner and less publicly, and he could have gotten over it faster. So yeah in the end, his pain is his own fault for taking her for granted.