Who was the howler for Aunt Petunia from?

Answer 1

If you don't want any spoilers for the Harry Potter series, I would advise that you don't read this.

Aunt Petunia's howler said, "remember my last", and was from Dumbledore. It was meant to be a reminder of the letter he sent her when he put Harry on her doorstep when he was one years old, and that letter said basically to take care of Harry as if he was her own son. However, Petunia did not care for Harry as her own and was prepared to throw him out when they thought Harry had attacked Dudley with the Dementors, but Dumbledore sent her the howler.

So basically, all in all, the howler meant to remind Petunia of Dumbledore's first letter and to not throw Harry out, as this is something she would not even consider doing to her own son.

Answer 2

Dumbledore sent the howler to Aunt Petunia.