um...is it??? from memory ( i don't even know where this is from and why it had implanted in my memory...but anywayz!) it's not ok to marry your first cousin but it is ok to marry others...don't know about legally tho, but apparently if you marry your first cousin and when you have a child, your child is most likely to have a defect of some sort :S
but don't worry, ayu is not yuuichi's cousin...unfortunately, my favorite nayuki is...