I imagined her as roughly the same "age" as Josie but maybe a bit smaller stature for some reason. There's nothing that suggested to me she was trying to "blend in" and hide the fact she is a robot. As for her mom using her to carry on for Josie - I figured the mannerisms, her walk, speech pattern, etc were more important than her physical resemblance but now you've got me wondering if she would actually be more physically like Josie than I imagined. Hmm...