Aqui estoy ... con misterio bonus

One more mystery:
Why my most confusing reading so far is the one in English.
(Each student in my Amazonian Ethnography class has to read two ethnographies assigned from a long list. Some were in English, some in Spanish, assigned without regard to the student's first language. A lot of anthropology work is done in English so upper level students are expected to read it.) This one just happens to be extremely technical:

"The unilateral cross-cousin categories complete a set based on the common Exogamous Group membership of ego's and alter's parents. If alter's mother is ego's father's Exogamous Group 'brother', then in choosing a unilateral term, the patrilateral link takes precedence just as it does in the case of bilateral parallel cousins."
What?

(Perhaps the problem boils down to the fact that I don't understand how mother's children are considered not siblings or how a cousin can be called unrelated)