Depending on the country, the era, the hospital and the prevailing mentality and procedures followed by the medical staff there, it is a possibility your twin was not viable ex utero. The doctors might have hidden it away and then spun the tale that the twin had been absorbed to save your family the anguish and pain.

Why not let a newly-blessed family go home with their one happy, healthy child and none of the pain?

The alternative is that your twin is was viable but was stolen away to be raised by someone else. But that sort of thing usually only happens in TV melodramas.