Sand Saref is an important character in the Spirit canon -- Arguably, Frank Miller stole Elektra's origin directly from hers (childhood sweetheart who gets traumatized by losing her father to violence, then later returns as an adult on the wrong side of the law). I'd be surprised if Miller didn't use those aspects of her character.

My guess is that she's being combined with P'Gell because Miller wants P'Gell in there as well, but the film is already overloaded with Femmes Fatale. I'm all for combining minor characters in adaptations, but not major ones, and these both loom pretty large...