Having an older brother makes a boy more likely to be gay! It's called Fraternal Birth-order effect, or 'older brother effect,' and recent science supports its claim.
The older brother effect is the strongest known biodemographic predictor of sexual orientation, and claims that the more older brothers a man has, the more likely he is to be gay. Scientists are not exactly sure why it occurs, but they believe it is due to a maternal immune response after giving birth to a son.
The mother's body reacts to the foreign H-Y antigens of her son, and creates antibodies to counteract them. These antibodies then remain in the mother's body the next time she gives birth.
Although this theory is not unanimously supported, it is generally accepted, and for sons with older brothers, can increase the likelihood of homosexual tendencies up to 48%!