Halakha does permit endogamous marriages, such as among cousins (I’d say that as part of ushmartem et nafshoteichem, there ought to be a takkanah banning these unions, due to genetic difficulties, as well as chillul hashem, since we view this as an Appalachian/backwards practice nowadays). It wouldn’t be unreasonable that a boy would have physical urges or impure thoughts towards such a cousin, especially if there was distance between them, and in such case, he should avoid idle chatter with them. But if the cousin is like a sister to him (and this is true in many families), I don’t see the harm in talking.
