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. Some people are even confused when Papa says his family is down to four after Fievel's supposed death because they forget Yasha exists. 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. She even sleeps through a scene where Tanya is holding onto her while sliding down a water spout. A normal baby would be either screaming or laughing hysterically. She is reduced to more of a prop than anything.
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. In short, her role throughout the An American Tail series seems to range between a prop and just to be there in the background to say 'look at me I'm a baby and I'm cute'.