Did Lily deliberately attempt to poison Petunia?

Question

In Harry Potter and the Methods of Rationality, we get this quote in Chapter 1:

"Anyway," Petunia said, her voice small, "she gave in. She told me it was dangerous, and I said I didn't care any more, and I drank this potion and I was sick for weeks, but when I got better my skin cleared up and I finally filled out and... I was beautiful, people were nice to me," her voice broke, "and after that I couldn't hate my sister any more, especially when I learned what her magic brought her in the end -"

Later, in chapter 17:

Dumbledore opened the book, seemingly at random, and Harry leaned in to see.
"Do you see these notes," Dumbledore said in a voice so low it was almost a whisper, "written in the margins of the book?"

Harry squinted slightly. The yellowing pages seemed to be describing something called a potion of eagle's splendour, many of the ingredients being items that Harry didn't recognise at all and whose names didn't appear to derive from English. Scrawled in the margin was a handwritten annotation saying, I wonder what would happen if you used Thestral blood here instead of blueberries? and immediately beneath was a reply in different handwriting, You'd get sick for weeks and maybe die.

"I see them," said Harry. "What about them?"

Dumbledore pointed to the second scrawl. "The ones in this handwriting," he said, still in that low voice, "were written by your mother. And the ones in this handwriting," moving his finger to indicate the first scrawl, "were written by me. I would turn myself invisible and sneak into her dorm room while she was sleeping. Lily thought one of her friends was writing them and they had the most amazing fights."

There is further discussion of this in chapter 119, but it's quite vague, and I don't see it as relevant to Lily's motives (although it does shed light on Dumbledore's motives).

What are we meant to infer from this?

The only inference I was able to come up with is that Lily either tried to poison her sister in cold blood, or experimented on her sister (depending on when that note was written). A "potion of eagle's splendo[r]" sounds like it would have worked just fine for Petunia without the substitution. But I'm pretty sure that was not the author's intent.

Answer 1

Assuming Eagle's Splendor works in the same way that it does in the Dungeons and Dragons game from which it was lifted, then the potion's effects are both limited and temporary, little more than a novelty. By adding Thestral blood (at Dumbledore's suggestion) Lily has apparently increased its permanence to the point that it's been active for decades.

Her first reaction is that it will cause sickness (and possibly death) but it's unlikely that Dumbledore would have suggested something that's actually deadly, implying that her first reaction might have been an overreaction, one that she later reconsidered, or at least decided was worth the risk.

Answer 2

My understanding is that Lily always loved and felt somewhat guilty for her sister, and was actively researching various methods to help her out. The potion of Eagle's Splendor could have been such a method, except for its very limited duration.

So when Lily discovered a note about Thestral Blood (which she presumed was left by her mysterious friend), her excellent knowledge of Alchemy allowed her to realize that this could be a possible solution to the problem: that changing a recipe in such a way would extend its duration considerably. She probably recognized that such change is very risky. She probably consulted with her Potions professor, and maybe even with Dumbledore. She even had an unpleasant conversation with a centaur (referenced in the first chapter):

And Lily would tell me no, and make up the most ridiculous excuses, like the world would end if she were nice to her sister, or a centaur told her not to ...

According to Petunia's recollection, it took Lily quite a long time to finally agree to give the experimental potion to her sister -- after Petunia literally threatened to end her own life.

And after she did, Petunia fell sick for weeks, and all that time Lily feared for her life. Luckily, it all worked out in the end. But when Lily came back to Hogwarts, she found her textbook with a note, and added her reply: "You'd get sick for weeks and maybe die", as a warning to her friend, or anybody else who might want to try the same recipe in the future...