[SuziQ]

No, Corin, you're not remembering incorrectly. "Remember my last" refers to the letter Dumbledore left with Harry on the doorstep, telling Petunia she needed to care for her sister's son until he came of age.

The letter Petunia sent to Dumbledore would have been long before Harry was ever born.

[Corin Stormhands]

Yeah, that’s it.

I think I was just expecting to learn more of that. But I guess Snape’s memory was just supposed to be enough for us to realize that Petunia did know what was going on.