The Genome News Network says here:
The same process also explains why siblings tend to be different mixtures of their parents' characteristics. For example, the chromosome 1 that you inherited from your mother might originally be the one that she, in turn, originally got from her mother, while your sister got the chromosome 1 that originally came from your mother's father. But perhaps you both inherited the same chromosome 1 from your father—the one he got from his father, say. And so on for all 23 pairs of your chromosomes—which is why you have your mother's nose and father's eyes, while your sister has your mother's eyes and your father's nose.
This confuses me because my understanding was the mutations cause these differences. How does my sister inherit a different chromosome 1 copy from our mother? Doesn't she (my sister) have the same chromosome 1 copy from my mother that I have?