I can explain in detail with Mendel's laws of inheritance. Your mother carries two copies of 'eye colour genes' one from each of her parents. Your father also has two copies of eye colour genes from each of his parents. Your parents each only pass one copy to you and your sibling.

this means that you may have a gene from your maternal grandmother that is not present in your sibling. And your sibling may have a gene from your paternal grandfather that is not present in you.