Okay, this kind of hit home for me. No, I'm not a woman (despite what people may tell you) and I don't have a twin sibling, but my great aunt did. I did some research on my family tree and I saw that my great grandfather's twin sister never married. She had been engaged but the wedding was called off. It was one of those family mysteries that will have to go unanswered. That's what I thought.
Scientists at the University of Sheffield found in 2007 that a male fetus's testosterone can have an effect on his developing sister. Testosterone and estrogen can cross cell membrane's and pass between fetuses. Male fetuses, however, aren't as effected by their sister's estrogen, as the estrogen level of a male fetus is the same as a female's.
Dr Virpi Lummaa at the University reviewed records of a Finnish church dated between 1734 and 1888 for their data. Seven hundred and fifty four twins were born in those one hundred and fifty four years. Of the females that lived to adulthood, the women were 25% less likely to have children if they had a male twin. They were also 15% less likely to get married.
There may be several reasons for this. Testosterone may have given the females a more male personality and attitudes about child rearing. Second, testosterone may have effected the female's reproductive development, reducing her fertility.
"The study used data from humans living in a pre industrial era so as to obtain results that are not affected by advanced health care and contraception. They showed that as a consequence of a male twin´s influence on a female´s fertility, mothers who produce opposite sex twins have fewer grandchildren and hence lower evolutionary fitness," says Doctor Lummaa.
Source:
http://www.sciencedaily.com/releases/2007/06/070619104000.htm
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