RooraSondazu, the explanation (in simplistic terms) is as follows:
Your mum must have two blue genes, because blue is recessive and if any other colour were present, that would dominate a blue gene.
Your father has brown eyes, but must have one gene for brown, and one for blue, the blue being recessive.
You get one gene from your mother - which is necessarily blue, and one from your father. It can be either blue or brown. If it is brown, it will dominate your monther's blue gere. You have blue eyes, therefore your father passed on his blue gene, and you and your sister both have two blue genes.