It is pretty hard to say definitively whether the fact that the men in the study are gay and happen to have older brothers is related to biological, psychological, or sociological causes, or is simply a random mathematical relationship that piques the curiosity of those who want to know why some men are gay and others aren't.
BTW: The only gay males I know of in my family are me, an only child of older parents; my father's first cousin, the only child of young parents; and my second cousin, the older brother of a straight younger brother, born 18 years later--but born between them was a lesbian sister. So go figure.