Googling around there don’t seem to be too many studies of homosexuality in twins. One seems to have found a roughly 50% matching rate among twins who were raised together. The methodology in that study seems to have been questioned a lot. But 50% in twins raised together seems to show there isn’t a strong genetic cause, since physically these are identical twins. And unless the twins were separated, we can’t know if they were exposed to an environment that led them both into this lifestyle. Certainly if your twin sibling starts doing something of his own accord, you might be inclined to imitate him, whether it be drinking, drugs or this.
The twin studies I am refering to were of twins seperated at birth.