We learned in my psychology class about a set of siblings in a similar predicament. Except this one was even weirder. They were only half siblings. Because genetic tests showed they each had different fathers.
I was a surgical tech for a while and we had to study this case when we were going through our OB rotation.
We also had to study different types of ectopic pregnancies. Ectopic just means outside of the uterus, it is most common
in the Fallopian tubes (called a "tubal pregnancy") - but can be anywhere in the abdomen. It can be very dangerous, because if the placenta "hooks" onto a major blood source (say the aorta) it can cause severe hemmorahing in the Mom. It is very rare though.