unless people have done human expiriments using the hormone, all you have is a correlation which may or may not support your claim. but you cerainly don't have proof and you don't have definitive support. you have a theory. and a theory with holes in it.
Whatever environmental factors lead to the first boy being gay were likely still in place when the 2nd boy was growing up... plus his older brother, a natural role model, is gay... that can seriously muck up the unbiased nature of such an observation. Even when multiple kids int he same family ARE gay, it does no more to suggest hormone deficiency than it does to suggest favourable environment. Only way THAT could be overcome is to switch babies with another family at birth and see which, if either turns out gay. And that is severely against all sorts of ethical practices. And momies antibodies don't seem a likely place to look for answers. if it's her own hormones she's stopping, then she has some serious auto immune problems. and if it's her son's... well, that makes less sense since antibodies won't stop him from producing the hormones and won't stay in his blood after birth.
Fact is we don't know WHAT causes it. Any theory is just that. A theory. And considering there is no proof suggesting it IS a hormone deficiency besides a hunch, then, no, you can't say for sure that's what causes it. Once the certain answer pops up in one of my medical journals, I'll believe you, but until then, you have no proof.
Cetainly all that link showed me was more skepticism of your theory considering how dead wrong it was.
I won't deny that it's possible that some genetic anomly could predispose someone to homosexuality, proof that that is the sole factor is severely lacking.