The crossword solution for the term TWIN, SAY is SIB, which stands for sibling. A sibling is a person who shares one or both parents with another person. Twins are two siblings who are born at the same time and share the same genetic material. Twins can be either fraternal or identical, depending on how many eggs were fertilized during conception. Fraternal twins come from two separate eggs that are fertilized by two different sperm cells, while identical twins come from a single egg that splits in two.