'Cellular Time Twins' - brothers and sisters of the same mother

Reason being the first cells to compose their body formed at the same time.  As long as they have the same mother, it does not matter who their father is, their first cells to arrive on planet earth come into existence at nearly the same time.  Although you have a brother or sister who is 20 years younger, if you have the same mom, your cell line is the same age.