Yasha only appears in the first few parts of An American Tail and is strangely absent towards the end, possibly a goof on the part of the animators. She was still present when the Mousekewitz family was being processed into America, Mama was holding her and Papa accidentally said 5 then changed it to 4 when he remembered what happened to Fieval. She was last seen about 27 minutes into the movie when she was asleep in her bed and Tanya was trying to convince their parents Fieval was alive.
By Fievel Goes West her appearance gets an overhaul just as Fievel's and Tanya's did, however, she still appears to be at least a year old, two at the very most (she doesn't appear to have aged as quickly as Fievel or Tanya, which is an enigma in itself). She is given a few amusing scenes in Fievel Goes West but she is in no way the main focus in any scene she appears in, and she descends back into obscurity for the direct-to-video sequels as a baby who is always bundled up and sleeping no matter what is going on.
But as strange as this is, she's the only character who's personality was actually enhanced in Fievel's American Tails, while everyone else's personality was mainly watered down or completely changed. Fievel's American Tails was where Yasha got her chance to shine. In the television series she is shown speaking (only one or two words at a time), and in the episode Babysitting Blues when Fievel is stuck babysitting her, she proves to be just as good if not better at getting into trouble than he is.