Cross cousin vs. parallel cousin
I'm confused, what is a cross cousin vs. a parallel cousin? I'm assuming it has something to do with the High Valyrian custom of marrying siblings (which I also take is why "brother" and "cousin" are often the same word).
In discussing consanguineal kinship in anthropology, a parallel cousin or ortho-cousin is a cousin from a parent's same-sex sibling, while a cross cousin is from a parent's opposite-sex sibling. So a parallel cousin is the child of the father's brother (paternal uncle's child) or of the mother's sister (maternal aunt's child), while a cross cousin is the child of the mother's brother (maternal uncle's child) or of the father's sister (paternal aunt's child).
The wiki page, under the heading "Kinship terminologies" also gives some real world cultures in which "brother" and "sister" are used for parallel cousins.