Cousins can get married?!
Remember how 兄弟 means "siblings?" Well when they're siblings you have to obey they're not your brother or sister, they're your cousin.
The entire reading for this is an exception. Sorry about that. Here's a mnemonic to help you:
What are the two defining traits your cousin has? They're good and a child. いいと子, or いとこ for short. Just remember your good, kid cousin. Your いとこ cousin.