I wonder why the sex of the remains is still not known. Could it be that the remains are of a very young petite adult so they're pelvic bone isn't fully matured yet? What is the normal of maturation for such? If a female started her period in her later teen years instead of earlier on, would that be a sign that her pelvic bone wouldn't be fully mature if she's in her young 20's?
http://anthropology.si.edu/writteninbone/male_female.html
This table outlines the differences between a male and female pelvis.
Male:
narrower, heart-shaped pelvic inlet
open, circular pelvic inlet
narrower sciatic notch
broader sciatic notch
narrower angle where the two pubic bones meet in front
Female:
open, circular pelvic inlet
narrower sciatic notch
broader sciatic notch
narrower angle where the two pubic bones meet in front
wider angle where the two pubic bones meet in front
more outwardly flared hip bones
Fact:
Sex-related skeletal features are not obvious in children's bones. Subtle differences are detectable, but they become more defined following puberty and sexual maturation.