Looking at it a bit later, it really wasn't a depressing ending at all. If you ignore the bait-and-switch of making it look like the younger sister was romantically attracted to the older girl, it ended up good for all concerned. After all, what younger sister wanted was a friend and for her sister to be nicer to her and she got both. Plus, both the others got a loving relationship out of it, so that's a good thing too.
Beyond that, the younger sister also got a wider perspective which opened her eyes to a lot of good things and I can't really call that a bad thing.