Simply a mother's love for her daughter and the conviction that such a harsh and exclusive attitude was wrong prompted Katrina to stand up for Kira. (Kira had a leg malformation, or something like a club foot.) She also prepared Kira for her future by transmitted to her daughter her weaving skills so sought after by the community.
When Katrina dies (actually, the reader learns later that she was probably killed), Kira's knowledge of the art becomes her livelihood. The irony of the situation, though, is that if Kira had not been so exceptionally gifted, perhaps Katrina would not have been eliminated so that the community could isolate Kira to exploit her talent.