wombmate
noun/ˈwuːm.meɪt/
Etymology From womb + -mate.
1
A sibling with whom an unborn child shares the womb.
- The chance to meet one's wombmate, possibly for the first time since the umbilical cords were cut, is irresistible.1999, Judith Rich Harris, The Nurture Assumption: Why Children Turn Out the Way They Do: