What does Per Stripes on a will mean?

"Per stirpes" means by the branch of the family, rather than "per capita," per person.  It means that each of the children of the testator (person who made the will) gets an equal share, and that if the child is deceased but leaves grandchildren, the grandchildren divide up their parent's share only, not equally with other grandchildren.

Unless your mother's will left everything to your older sister and nothing to you, there isn't anything in your question that explains to me why you got nothing.  Please talk to a local attorney about this, as soon as possible, for a more detailed look at the facts of this case.  You may be able to do something about this.