In males, it appears to be that number of older brothers alters the likelihood of same-sex preferences later in life. Specifically, more older brothers (not sisters) is associated with homosexuality in men (not women). This is called the fraternal birth-order (FBO) effect in the scientific literature, and the evidence for the FBO effect is compelling. Simply, homosexual men, on average, have more older brothers than do heterosexual men, a difference that is not seen in homosexual versus heterosexual women.
A logical response to this finding would be to wonder whether growing up with older brothers somehow led more men to develop with same-sex sexual preferences, or if there was something about the uterine environment that favored homosexuality in successive male offspring.
Reflection Question: How would you address these differences scientifically?
For an accessible summary of the FBO effect, read: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1502267/pdf/zpq10531.pdf
Caution: These differences in uterine influences on male and female homosexuality also illustrate a key point: male homosexuality and female homosexuality appear to have different causes, thus we should be careful not to transfer the findings of research on men to the reality of sexuality in women. Further, male and female homosexuality are likely influenced by multiple factors.