There was a study quoted that hormonal and other influences on the fetus while still in the womb may have an effect on sexuality.
There was a link to information about identical twins showing that they do not necessarily receive the same nutrients.
That you claim there is no evidence shows, as Damo said, "...you have the ability to ignore inconvenient evidence when you want in order to maintain your preconceived notions".
The two studies shown are more evidence than you have offered. Your "usually the simplest explanation is the right one" is not any sort of evidence.
So far you have maintained that both studies offered as evidence are wrong, and that environmental influences beyond their control are nonexistent.
In other words, you want to be right so you claim you are right.
It is just as likely that the straight sister is a latent lesbian, but has been scared by the social conservatives into pretending that she is straight. Given the amount of bias against homosexuals, that would be the simplest answer.