Androphilia refers to sexual attraction and arousal to males whereas gynephilia refers to sexual attraction and arousal to females. In Samoa, transgendered androphilic males are known locally as fa'afafine. Previous research has shown that, compared to Samoan gynephilic men, fa'afafine report greater willingness to invest time and money toward nieces and nephews (i.e., greater avuncular tendencies) and also have greater numbers of older brothers and older sisters. The present study examined whether the Samoan male sexual orientation difference in avuncular tendencies could be accounted for by these parallel differences in numbers of older brothers and older sisters. The sample included 204 fa'afafine and 272 Samoan gynephilic men from our Samoan data archive for whom we had concurrent information on (1) a measure of willingness to invest time and money in nieces and nephews (i.e., avuncular tendencies) and (2) numbers of older and younger biological brothers and sisters. Among fa'afafine, but not Samoan gynephilic men, number of older brothers and number of older sisters were both significantly positively associated with avuncular tendencies. When controlling for number of older brothers, the magnitude of the male sexual orientation difference in avuncular tendencies was lowered, but remained statistically significant. In contrast, when controlling for number of older sisters, the male sexual orientation difference in avuncular tendencies ceased to exist. Discussion detailed how these findings help hone in on the proximate basis of elevated avuncular tendencies among fa'afafine. In addition, discussion focused on how particular evolutionary and cultural factors might relate to the avuncularity of fa'afafine.