This reminds me of the rather silly British idiom "And Bob's my Uncle."

From the Washington Post:
According to a study in 1998 1 in 8 births start out with 2 fetuses, but only 1 child is born. The result occurred recently in the case of a Caucasian Washington couple who donated some sperm, and when the wife was impregnated the child did not have the blood type of either parent. It was assumed an other person gave it. However, there was only 1 white male sperm donor that day when the sperm was given. Geneticist Barry Starr explains: there is a 50-50 genetic relationship between parent and child. There is a 25% relationship between uncle and nephew/niece. As it happened this man had a brother in utero, but the brother's dna merged with him as the 9 months of birth occurred. The man's dna is 90% his and 10% his unborn brother. The result of the sperm test was that the brothers DNA was the source. Thus the child's father is the dead uncle, and the father is an uncle.