This quote has no mention of a deity (divine authority) or society as a whole (civil obedience), and the only mention of gender is the author's reference to his sibling as "brother." The final line "I won't be caught betraying him." drives the assumption that failing to do his "duty to [his] brother" would violate some sort of bond between he and his brother. Since we only know this relationship as familial, we must assume familial loyalty is the theme.