I wouldn't call it "theft," not really. It's a re-imagining. The only thing I'm ambiguous about is the way that Amara never once acknowledges her source material. I think that's only okay to pull off if you can assume that your audience already knows the source material. I.E., neither Rent nor Moulin Rouge ever once explicitly acknowledged "La Boheme" as their source material, but they didn't have to, because anybody with any sort of interest in musical theater would have recognized that connection instantly.

The Archer's Heart is different, though, since it seems to be aimed at an audience unfamiliar with the source material. In that case, I'm still not sure how I feel about the non-acknowledgement of the Mahabharata. I already read one review of the book that seemed to assume that Amara had wholly created the world and characters on her own, and that's really not true. So. Er. I dunno about that.