The fraternal birth order effect is a phenomenon that can be described in one of two ways: Older brothers increase the odds of homosexuality in later-born males or, alternatively, homosexual men tend to have more older brothers than do heterosexual men. After statistically controlling for number of older brothers, homosexual and heterosexual males do not differ in their mean number of older sisters, younger sisters or younger brothers. Older sisters, younger sisters, and younger brothers have no effect on the odds of homosexuality in later born males – they neither enhance nor counteract the fraternal birth order effect.
[T]he fraternal birth order effect operates through a biological mechanism during prenatal life, not during childhood or adolescence. Direct evidence for this is the fact that the fraternal birth order effect has been found even in males not raised with their biological brothers – it has been determined that biological brothers increase the odds of homosexuality in later-born males even if they were reared in different households, whereas non-biological siblings, such as step-brothers or adopted brothers, have no effect on male sexual orientation…
Anthony Bogaert’s work involving adoptees concludes that the effect is not due to being raised with older brothers, but is hypothesized to have something to do with changes induced in the mother’s body when gestating a boy that affects subsequent sons.