As requested!
I think overall this query makes your story more compelling, and I definitely think distinguishing between the Seb she has a crush on and the alternate evil Seb is important (I was confused before, thinking it was the same guy but traveling around and following her, and I wasn't sure how she could have a crush on him...) Anyway, I think you cleared that up, so

I like the inclusion of some of the details of the other worlds, and how she came to have the pendent. There are a couple of sentences that were a bit tough to read, so I'll make more detailed comments below.
Cheers and good work! I think this sounds like an awesome girls MG book. I always thought there were so many MG "adventure" books/series for boys, but not as many for girls (we need some fantasy versions of Fancy Nancy and Judy Moody!) So, I seriously wish you the best of luck with this.
Hi, Everyone has been so helpful.
I think I might be ready to send this in. Any last thoughts before I do?
Again, thank you so very much!
An alternate version of herself gives twelve-year old Amelia a pendant allowing her to travel between parallel worlds.
This sentence reads awkwardly to me, but I think it's cool to say she gave herself the pendent. I think it's awkward because the first character introduced is actually "another" Amelia. How about something like - "Twelve-year-old Amelia can travel between parallel worlds thanks to a pendent she receives - from another version of herself." At first, she is only interested in visiting her dog, who is still alive in the
serious academic-minded reality of the
serious “Amelia” who gives her the pendant.,
End the sentence here, otherwise I think it's too long. but she eventually Her curiosity takes over though, and she visits four other realms; one where people live in underground cities
so the earth’s surface remains unspoiled, an old-fashioned world built up around geysers and steaming mud pits, another that is book-obsessed and almost entirely covered in water, and a world where robots do all the work while humans play
and her middle school’s motto is “all fun all the time.” The only reason I chopped out parts of this sentence is because I felt it was getting long, and you gave some good information already. The suggested trimmed down parts just reiterated what you already said about those worlds. I definitely think this short breakdown of other worlds is worth keeping in though. I felt like I wanted to visit those worlds with her now, much more strongly than I felt in the other query.Soon she is She is sooncaught up investigating catastrophic events
that are killing people in these other worlds, such as a super solar flare and massive earthquakes
killing people in other worlds. One version of her family dies as she tries to save them
This is the "stakes" part of your story, so I think for dramatic effect this should be two shorter sentences. and i If she can’t figure out the cause of the disasters, the next realm that is destroyed may be her own. But how is she supposed to do that when every time she travels to a parallel world, an alternate “Amelia” takes her place in her home reality, causing problems, getting her grounded, and telling her mom she’ll try out for the middle school cheer team?
Complicating everything is her friendship with Seb, a boy she keeps meeting in the parallel realities. She has a little bit of a crush on
This may be nitpicky, but I think you could insert some of her "voice" here. Something like "She has a little bit of a crush on him - well, one version of him (or, "well, the cute version of him in XXX world"... whatever real Amelia would say) - but learns that an alternate Seb is the one causing the deaths of millions one version of him, but learns an alternate Seb is the one causing the deaths of millions. He killed everyone in the alternate realm of the last person who tried to stop him
, and now Amelia must put her own family and friends at risk if she attempts to save the world(s)
I don't think you should bracket the S. Just say "if she attempts to save the worlds"... it may not sound grammatically correct, but it IS for this story. I think it's another subtle zinger. 
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Written in the spirit of the PENDRAGON series, AMELIA AND THE MANY WORLDS is a middle grade fantasy, complete at 63,000 words.