Wow my brain is reeling with possibilities now. What could she mean, why would DD be writing to Petunia? We already know she has no wizarding power and she is in deed a muggle, but perhaps there's something more to this? We can only assume that he was referring to the magical protection he left with Harry when he left him on the doorstep that night, that is to say the power of love which protected Harry and, in deed his whole family at number 4 Privet Drive. Assuming this was the case, he was reminding Petunia that as long as Harry stays with them oer the summer, nothing can happen to him or them.

Because we all know that Petunia wanted nothing to do with the Potters or their son by the way she almost went crazy during SS/PS at the mention of their name. And every year she became more adamant about getting Harry out, not letting him read news papers, or watch the Telly with them. Was she hiding the fact that she was secretly listening to any signs of the Wizarding world, secretly hoping she would spot a relative of Harry's even he didn't know about? Maybe DD was sending her reminders of the pact she made by allowing Harry into her home, and reminding her to keep him healthy and safe.

But let us assume it wasn't about that. Then, we must think of why else he would be writing to her in the first place. Is there somethign we don't know about hre? Obviously that is the case as we don't know much about the Dursleys, or weither there is more to them than meets the eye. It wouldn't surprise me if Aunt Marge has some magical abilities and that DD placed a protective charm around her and her family, in cordance with Harry living wih the Dursleys.

Now, if that's the case then it would not surprise me at all.

That's my thoughts on the subject