The Arabs prefer endogamous matrimony -- that's marriage with first cousins or second cousins. (In fact, refusal to marry cousins (or regret at having done so) is one common reason that many Arab Muslim women become victims of "honor killing.")
In most societies - thoughout all of human history, most people think that the "in-law" relationship is the most troublesome in their lives; (it's one of the most popular and enduring sources of humor; America's most popular TV comedy in decades - EVERYBODY LOVES RAYMOND - was based on it). WELL: Because of the preference and prevalence of endogamous matrimony many Arabs are so inter-married that NOT ONLY are they everybody's in-laws, every Arab is his own in-law! No wonder they're so effin' nuts! Heh!