This part is true. I read the study.
". . . for every older brother a man has, his chances of being gay increase by one third. Older sisters make no difference, and there's no corresponding effect for lesbians. A first-born son has about a 2 percent chance of being gay, and the numbers rise from there. . ."
I have two dogs who are brothers, they've been together since birth. So, they've had the same environment. One is the "alpha" dog, the other not. They're as different as night and day.
That doesn't mean the differences are genetic...we can grow up in the same household but have a totally different experience.
Let's don't get too far away from the fact that, as yet, we only have the word of the self-declared that they are gay which is the only scientifically valid reason for doing studies such as these.
Prison sex or any group sex act has an element of coercion.