Why is my ancestry not identical to that of my sibling? 

At the genetic level, siblings share 50% of their DNA. Both receive information from the father and the mother, but the reception of this information is random. In the same way that there are siblings where one of them physically looks more like the paternal side and the other more like the maternal side, the same thing can happen in the ancestry DNA test results. Thus, one can observe the ancestry of the father more represented in one of the siblings and that of the mother in the other, giving rise to differences between the results of both.