No matter how I look at it, that pairing just doesn't work for me. It's not even about the age gap, Sakura just never seemed to show much interest in anyone from the group, she's feeling like less of a character in the story, than Rin's grandpa, so it's really hard to justify that kind of development.
Rin also feels completely unlike herself in the doujin. She would have never even tried to ride through the mountains in the rain, to begin with, so it kinda breaks their whole initial interaction. Same with the "I'm starting to hate myself" comment - completely out of character. Doesn't really affect the core story, but pulls you out of it from the very beginning.