1) Definitely ask your local children's librarian for help! They know what is really popular with kids of all ages, and can direct you straight to the right shelf. Best of all, libraries are free to use! You may even be able to search your library's catalog online from home, or have your sister read them digitally through Overdrive or another digital library platform.
2) These are all Amazon links, but only so you can read about them. They are not affiliate links, and I suspect you aren't interested in actually purchasing them anyway.
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Your sister may be interested in:
Dinosaurs
><em>Dinosaurs Before Dark (Magic Tree House)</em> by Mary Pope Osborne
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>The entire <em>Dino Rider</em> series (my 7yr old loved it!) by Will Dare
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>Anything from this Educator-curated list, or this more science-oriented dinosaur list
Snakes
><em>The Girl Who Wore Snakes</em> by Angela Johnson (look for it at the library)
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><em>Kali And the Rat Snake</em> by Zai Whitaker (look for it at the library)
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><em>Verdi</em> by Janell Cannon
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><em>Miles and Miles of Reptiles: All About Reptiles</em> by Tish Rabe
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><em>Wild Reptiles: Snakes, Crocodiles, Lizards, and Turtles</em> (Wild Kratts) by Chris Kratt
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><em>Everything You Need to Know About Snakes</em> by DK
Bats
><em>National Geographic Readers: Bats</em> by Elizabeth Carney
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><em>Oscar and the Bat: A Book About Sound</em> by Geoff Waring
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><em>Bats</em> by Gail Gibbons
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><em>Bats at the Beach</em> by Brian Lies (Book 1 of 4 in the Bat Books Series)
First thing that came to mind, that might be close to that is this, https://www.amazon.com/Dinosaurs-Before-Dark-Magic-House/dp/0679824111
maybe its Dinosaurs Before Dark. Its a magic Treehouse book.
http://www.amazon.com/Dinosaurs-Before-Dark-Magic-House/dp/0679824111