Halachically, first cousins are permitted to marry each other, hence they have all the regular Dinei Rrvah and then some. Even HaEzer Siman chuf-beis states that relatives have the din of �Libo Gas Bo�, meaning there are some hateiri of Yichud that while applicable in most cases are not applicable for cousins, who have more restrictions than normal. Shulchan Aruch (E.H. 22:8) and the Aruch Hashulchan (E.H. 22:6) present examples of a man and a woman who grew up together or are related � such as cousins.
