Brothers sisters have you none but that mans father is your father?

your grandfather is the relative which is described

- a) (If I am a female) your grandfather is my father's son, therefore I am your grandaunt (your grandfather is my brother)

- b) (If I am a male) your grandfather is my father's son, therefore I am your granduncle (your grandfather is my brother)

- c) (but if I am a sole son/daughter) I am your grandfather/grandmother