Here are a couple of ways you can make the book harder without drastically changing the content (because it sounded like your sister loved the book story-wise).
Think deeper about your theme and plot
This does not necessarily mean to add plots or sub-plots. Instead, consider the origin of your plot, where you got inspiration, and how you based your storyline on it. Now, take that base of your story and stretch it.
Stretching your base would mean growing where your story comes from. Your base is similar to your original theme. Think deeper about your theme, what exactly is it and how do you display it? Try and incorporate that where you can in your story while still avoiding predictable tropes.
Thinking deeper about your theme and making it an underlying part of your story could make your reader also think deeper about the origin of the plot and the more subtle theme.
Now, when I say to bring out your theme a bit more, I don't mean making it like Goldilocks and the Three Bears; making the theme the entire story (don't break and enter, be nice, treat others property well, etc.)
I mean to go into greater detail when describing theme associated parts of your book to let your reader think deeper about the theme and possible hidden meanings.
So, to sum up this tip, thoughtfully bring out your theme without over exaggerating it.
As writers, if we're not careful, sometimes we make our sentences very generic. Each sentence starts to sound the same except for different words. Try other formats of sentences! It makes it more fun and challenging to read.
If all the sentences have similar structures, I can see why your sister would be able to skim through the book.
It is important to try out different lengths of sentences too. Maybe break up some of the "ands" into individual sentences? Always vary conjunctions and stuff like that to create more engaging sentences.
You can practice this by writing a whole chapter speaking like Yoda. "Feels strange, it will."
So, to sum up this tip, vary sentence structure and length to create a more interesting and challenging read.
Throw in some unique vocab
One way not to do this is to just use a thesaurus on average words. Your goal is not to completely throw a word in to stumble your reader. Your goal is to put the perfect word in to smooth out your story. Sometimes, your readers will have to look up the word, but as long as the word is the perfect word, then it is fine.
Research your proposed word and make sure it fits perfectly. Ask for help from beta readers or friends if you are unsure if the word is truly perfect for the given scenario.
Give your characters depth
Strengthen your characters to make a more challenging read. Give them depth. Stronger emotions. Your reader will have to keep track of detailed, emotional, and real characters. I for one definitely think a book is more challenging when I have to figure out a character's complex emotions.
Not only will you succeed in making your book harder, but you will also secure a more memorable read. It is easier to remember something if there were strong emotions tied to it.
You can accomplish this by drawing out emotions in your characters over significant events. You can also give your characters flaws to do this as well. The more real a character seems, the larger the impact on the reader.
So, to sum up this tip, fill in your characters. Give them complex emotions to occupy your reader more.
Improve your dialogue
Don't just write dialogue to get a point across or to fill up an empty scene - write it because the characters need you to. Pour emotion, pour love, pour your skill into your dialogue.
It needs to sound real to be believable. If the dialogue is soulless, your reader will easily fly through it.