Wow! I love it! I personally think that the red background was a good choice because red is more of an emotive color... But I think that horse is so awesome that it would look good on any background color! Haha
Beautiful, absolutely stunning your sister is one very talented girl!
The red works perfectly with the bay, I do the same thing when i work in pastel, using the paper colour to draw out a colour in the coat of the subject. I use blue for chestnuts, because I use so much blue in the shadows and it draws it out, people think it is a strange way to go, but it pulls the drawing off the page so much better than putting it on a black or white background.
Your sister certainly knows how to use varying colours in a work.