Milkette is a clone.

Milkie, a male mouse, once experimented with gender-swapping. Stepping into a machine that changed genders, the machine malfunctioned and instead replicated his DNA. It made the necessary additions of female hormones, chromosomes and other things to alter one's gender from male to female... and Milkette was born. She now lives as a separate entity, apart from her 'brother,' Milkie.

Living as a female clone of another presented its own challenges at first, such as being hidden from the public view. But after she was initially discovered by Milkie's family and reluctantly accepted, she was able to more comfortably slip into society under the guise of being Milkie's biological twin sister. Many wouldn't believe she was a scientific blunder in any case. Since then, she has worked towards a feeling of independence, taking her life, a replica of another's, and making it her own.

Art (c) Mizers, who is on FA here: http://www.furaffinity.net/user/mizers