Hmm, first thing first... Greek or Roman?
Well, in Roman mythology, Cupid was believed to be the son of Mars and Venus...
On the other hand, in Greek mythology, Hephaestus and Aphrodite were husband and wife, while he and Ares were brothers. Eros (the Greek version of the Roman Cupid) was widely believed to be the son of Ares and Aphrodite...
If you really want to probe further, try searching in Google if there had been a DNA test conducted... :p