He was probably conditioned to their behavior, all the time that he was growing up with them. His defense mechanism was probably just tuning them out. You're an outsider (in a sense) to his family, because you didn't grow up with them. So you could see that the women (his siblings) were lunatics, right off the bat!
Family dynamics are tough to deal with, let alone even try to figure out. He's between a rock and a hard place with those gals. He probably does the minimum around them, just to keep in touch with the family. The more people I know, the more I find it difficult to find people whose family is not, in some way, dysfunctional !!