In underworld 2, selene was already born when william (markus bro) was imprisoned as she held they key to the prison as a child. But during underworld3 , lucian takes the key from sonia after she dies and holds onto. that would indicate that selene was born when underworld 3 took place. However, in underworld 2, selene doesn't know about lucian, sonia and only gets this info from Tan-is.