All the way back in chapter 1, Petunia says:

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.

The main (although not the only difference) between canon and HPMR is Lily and Petunia's interaction in this context. In canon, centaurs are creatures which see the future. This makes me very worried that a centaur foresaw that if Lily helped Petunia bad things would result. Since this is the main departure from canon, is this a reason to think that the story is going to have a really downer ending?