When Dumbledore sends Aunt Petunia the howler that says, "Remember my last" is he referring to the letter he left with Harry as a baby? If so what in the letter is he referring to?

I believe he is referring to the letter he left with Harry, and in that letter, didn't it say something about how Harry would be protected in the place he called home? I think that's what it's referring to, and Petunia realised that even if she hated him, she had to allow him safety, at least for a little while.

^^ I agree. He was referring to the letter he left with Harry. But maybe there was something in the letter that we don't know about. apparently something threatening if it scared petunia.

I think that when she got the first letter it said to love him, take care of him as if he was your own but was always treated as an outcast and sleeping in a broom cuboard. Maybe that is what he is refering to.

Dumbledore had included in the letter that they should love harry, and other stuffs, but perhaps the last part, the last instruction in the letter is that they had to let Harry stay in their house until he reach the age of 17 and be an adult in the wizarding world.

I think basically he is saying "Remember my last words to you", meaning the letter that he left on the night he left Harry on the Dursley's doorstep. It is this that makes Petunia quickly change and insist that Harry stays, because of what was said in the letter about the protection charm placed upon Harry until he is 17.

I think he is refering to Lilly's last will. I mean why would Petunia do anything for Dumbledore. He rejected her petition to go to Howarts. She hates him beacuse he stopped her form becoming a witch. Lilly's last will seemes to me the most resounable option, after all, she was her sister, even if they didn't speack to each other much

Personally, I don't think it was referring to making sure they loved Harry (you can't force someone to love, right?) because of earlier when, like some said, he was living in a cupboard. I think it was just referring to the fact that his "last" (the letter) described the blood protection. And maybe it wasnt a threat, but more a conscience thing. Like, she would know what would happen to Harry had he left (or the Howler made her realise it was serious).

I don't think there was much more than an explanation in the letter. But, for all I know, there could be.

Yes, I do believe he was referring to the letter he left with Harry on the doorstep. I think Dumbledore was reminding Petunia that while Harry could still call 4 Privet Drive home, he was still safe under his mother's protection. Also, as we learn more about Petunia in DH, she wanted to be a witch as well, because she felt left out with Lily's being a witch. She wanted in on the wizarding world, and so she must have learned much about it. So Petunia knew about the magic that protected Harry so well.

I think it was referring to the letter, yes, but as LoonyMagic had said, it was talking about the last words of Dumbledore. Probably an agreement of giving Harry a "home" with his mum's blood.

Dumbledore told her to take Harry in their family and they never treated him like family.