Cain and
Abel (so tongue-in-cheek, but I do have a brother
Kain and love
Abel enough to use it on a son)
Aurora (dawn) and
Layla (night)
Blanche or
Bianca (white, fair) and
Melanie (black, dark)
Cecilia (blind) and
Aisling (dream/vision -- not quite an opposite, but it works)
Liahona (compass) and
Diggory (lost one)