I am a geneticist who did some research a few years ago regarding this topic and can say that there is no published data regarding the percentage chance that a baby will be born and end up gay. However, that are some correlates that could indicate an increased likelihood of someone being gay. For example, it has been correlated that the gay men have generally have more relatives, specifically paternal relatives, than straight men and that they, in general, have more gay relatives.
Furthermore, in line with current knowledge, the strongest predictor for male homosexuality is a concept known as "fraternal birth order." This phenomenon states that a male's chance of being gay increases as the number of older brothers (not sisters) increases. Male fetuses produce an immune response in the pregnant mother that, to some extent, regulates sexual differentiation of the fetus. Consequently, the more older sons the mother has, the greater the immune response will be in the unborn child further increasing the likelihood the boy will be gay.
As far as the twins questions, there is absolutely no evidence to suggest that half, or all, twins will be gay.