"Ron had a full dose of the infamous Weasley temper from his mother's side of the family" Ron's mother was a Prewett so therfore he could not have gotten his "Weasley Temper" from his mothers side of the family.
Author's response
Is Mrs. Weasley a Weasley? Yes, by marriage. Is her temper well-known? Yes. Is his mother from his mother's side of the family? Yes. Then the infamous temper came from mother's side of the Weasley family.
I don't recall anything in canon to suggest that any previous Prewett had such a temper, though to be honest I don't care enough to look it up.