Nope, that's just Buffy's unmentioned long lost aunt. Like the identical cousins from Patty Duke. Darla might have been reincarnated more than once and turned, only to who she really is, same as Angel was reinsouled more than once.

Absolutely already saw that coming. It was the only thing that explained why they were so desperate for her egg after her mom tells Leland "You know how many demons I dated before you??"