Twin Family

Identical twins look alike, although they do not have the same fingerprints (which are environmental as well as genetic). As they mature, identical twins often become less alike or more alike because of lifestyle choices or external influences. The children of identical twins would test genetically as half-siblings rather than first cousins.
Semi-identical twins
Half-identical or semi-identical twins (also referred to as "half twins") are the result of a very rare form of twinning in which the twins inherit exactly the same genes from their mother but different genes from their father. Although examples of half-identical twins have been found, the exact mechanism of their conception is not well-understood, but could theoretically occur in polar body twinning where sperm cells fertilize both the ovum and the second polar body.