"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 - the most ridiculous things, and I hated her for it."
A centaur told her not to.
A centaur told Lily Potter not to help her sister.
A centaur told Lily Potter that the world would end if she were nice to her sister.
Because that centaur was afraid.
Because the centaur knew what sort of child Petunia would raise.
Because the centaur knew what would happen when that child finally grew old enough to attend Hogwarts.
Because the centaur knew that once that boy set foot inside the castle, nothing would ever be the same again.
The world as we know it...
Here is the link to the story.[link]