An Elder is a being born before the energy released during the Fall changed the world and it's inhabitants, but after Danu Talis was raised from the sea bed (presumably releasing similar energies which altered the procreation of Great Elders into Elders). It makes sense Hel, as an Elder, would have Elder family since there was a long period of time between those two events in which the Elder race flourished.
Hel has been confirmed as an Elder multiple times, I'll find a quote when I get home.
If Hel is Mars' niece that means she's also Bastet and Hekate's niece and Aten, Anubis and the Morrigan's cousin. As well as the grandchild of Asteria and Hyperion. This would also make Odin the brother of Hekate, Bastet and Mars. Interesting family tree.
KInd of off-topic but there's no place for it, Hekate called her parents Titans. Has it been mentioned if Titans were one of the primordial races like Earthlords, Ancients, etc?