Congrats on making your first website for a client.

The site looks indeed clean and seems to "respond" well (as in RWD).

A couple of suggestions:

• The website looks too generic and simple. Simple as in "flavorless", not minimalist. Your sister's work is very nice and colorful, and you should try to somehow reflect this to make this website a good companion to her portfolio. Maybe adding some texture details or bigger images (not just photos of the illustrations, but the illustrations themselves).
• Not a big fan of carousels, but if you include one, make it more useful. For example, clicking on a thumbnail should take you to that specific project.
• I personally dislike having to hover over an image to see its description (like on the Portfolio page).
• I'd ask your sister to include a description of each project. What she did, learned, and some sketches.
• I was going to suggest optimizing your stylesheet, which right now is huge, by removing all the Bootstrap styles that you aren't using, but, assuming you're a young developer, I think you'd benefit greatly from re-developing the website not using any frameworks.
• Images. You may want to host the images on your own server, or any other reliable/trusted service. Imgur is fine for casual posting on the web, especially forums, but since it's free, they could remove your images without notice or take its service down without you having any customer support.

Simple, clean and organized. Great job! Some good advice in the comments, but if your sister is happy with it, then mission accomplished! Best of luck!

Why is the Blog just her instagram pictures? Other than that, I agree with Raffaello about trying to bring more of her personality/style into the overall design (without over doing it).

Why use the carousel to go sideways instead of going down? I feel like there should be more on the page when I scroll down. I would drop the carousel and instead arrange all the images underneath each other.

Really nice and clean!

Couple of thoughts:

• Half of the work sample row is cut off by the "fold" on my 13" MBP on the homepage. Consider cutting down the vertical whitespace used in the nameplate on the homepage or Use that space for an additional product shot, and overlay the name on top or form a grid where there's a product image tile both to the left and right of the name, but not behind it

• On your contact page, there's the opposite problem - it needs more vertical room above the header (make it consistent with the other pages)