If you notice, Riley seemed to have the greatest control over the sensate powers than any of the others. She had the most interactions with the other characters, and she seems to draw them in and focus them (the singalong caused by her listening to music, all of them remembering their births because of her, all of them being together on the boat at the end)
that's a good point-she's almost like their "core" in a way, or their mother.