Curves! Hallelujah!  Someone fed the poor waif!

The hip/thigh position is a bit past realism, but trust me, there are people who can torque their joints nearly that far however, so if the lady happens to be hyperflexible it isn't that far out of the possiblities of reality.  Actually, if the knee were brought forward a bit and the calf/foot adjusted to compensate it is entirely possible.  Bending the straight leg a touch at the  knee would help too.

The leg pose is very reminiscent of some is Indian and south east asian forms of dance.