Since the mother already have a lot of estrogen in her blood, it might simply be that the effect of a female twin does not impact the amount of circulating horm

Since the mother already have a lot of estrogen in her blood, it might simply be that the effect of a female twin does not impact the amount of circulating hormones. The effect of increased estrogen in the male twin similar to the increase in testosterone in female twins would be interesting, but it might just not be something that can occur naturally. Thus if I speculate a bit, the reason for the non-symetric effect should be that the mother and father does not have symmetric roles during the pregnancy. If half the time it was the father that carried the child we would likely have seen a similar effect on males with female twins. What would be interesting is if they looked at mothers with different levels of estrogen and see if that effects male offspring, in particular those with exceptional high and low levels.