Genetically? Yes. It would also mean your grandfather is also your father. If your grandfather had an incestuous relationship with your mother and you were born as a result, it would mean that your parents are actually your mother and your grandfather. And being that your mother is also a child of your grandfather, as you are, it would make your mother your half-sister (same father, different mothers).

Now in a slightly less genetic way, my brother had a friend growing up who had an older sister. Turned out that his sister was actually his mother. She got pregnant at 13 and had him and since she was unable to raise him alone, her parents (his grandparents) adopted him and raised the two of them as brother and sister. Genetically, they are mother and son but socially they are brother and sister.