I'm currently in the middle of writing a story, and one of the story elements threw up a question for me. In this story, there are two siblings, who are only one or two months apart in age. As they grow older, they start to question how this age difference is even possible. How could their mother have had another child after just a month of giving birth to the first child? They suspect that they're not related and don't actually have the same mother, and it turns out that it's true.

I want to know if that suspicion is justified. I began to think of ways two siblings can be less than the usual nine months apart in age and yet still be related to each other. One idea I had was of twins, one of whom is born earlier, while the other had to stay inside the womb for another month for whatever medical reason.

Basically, my question is: can you have siblings (with the same mother and father), who are less than nine months apart in age? And if yes, how?

What do you mean by siblings?

If by siblings, you accept cases of individuals having the same father but not the same mother, then of course, it is possible! Below, I will assume you are referring to full siblings (eventually twins).

Age gap between twins

According to the huffingtonpost, there is a case of two twins that were born 87 days apart. On average, the age gap between two twins is rather of the order of 15 minutes (Rayburn et al., 1984).

Cousins that "look like" full sibs

Imagine family A has a pair of homozygotic twin daughters. Family B has a pair of homozygotic twin sons. If the sons of family B mate with the daughters of family A, then the offspring will be cousins but will have a relatedness of two full siblings.

One can of course extend this kind of crazy scenario to any number of generations apart.

Superfetation

I discovered the concept of superfetation in @froimovi's answer. From wikipedia


  Superfetation (also spelled superfoetation and superfœtation – see fetus) is the simultaneous occurrence of more than one stage of developing offspring in the same animal. It is not believed that it occurs naturally in humans. There have been 10 reported cases of possible superfetation in humans.


As @1006a rightly pointed, the two babies might well be delivered during the same labour though. If they are delivered during the same labour, then they will have the same age counting from birth but different age counting from the time of fertilization.

Uterus didelphys

See @Bakuriu's answer!

Human intervention

Other alternatives would require some human intervention. For example, fertilization could have happened in vitro and the eggs were implanted in two different wombs. One womb could be a surrogate mother and the other one could be either another surrogate mother or the woman who actually donated the ovules.

Plenty of cloning techniques could yield two full siblings to be less than two months apart. Also, techniques of ex utero pregnancy could be used (although I am not sure we have the technology ready for that). Note that human cloning is illegal in many countries.
