I only recently started to like double exposure photos, and I think this one is very good. As you said, your sister is beautiful. In my opinion, all the colors compliment each other very well. The flower in her hair was a very nice touch, bringing the two images together. As for improvements, I don't think there is anything that would make this photo better.
Very beautiful. Everything works together very well. I can't tell where her face begins and the landscape ends! It doesn't at all feel like you just dropped a picture of a face on top of a picture of a landscape--everything blends and mixes smoothly so that it looks like one piece of artwork (something I've seen a lot of others struggle with). Great job going outside of your comfort zone!
I think that you chose two photos for this combination which fit extremely well together (concerning colours and atmosphere...) Is it digital double exposure or id you do it straight on your camera? And if you did, how did you do it? I'd love to try out double exposure as well, but I haven't found any tutorial which describes how I could do it with my camera :/