In the beginning of the story, Lily's anger was described as palpable because it was
palpable (adjective): easy to feel.
At the beginning of the story, we are told that Lily’s anger was palpable. The story goes on to describe how although Lily and her brother both wanted a bike for Christmas, only Lily’s brother got one. This makes Lily angry. It can be well understood that it is expected of a young girl like Lily to react in this particular manner. Hence nothing unusual, irrational or difficult to understand here.