In Volume 3 as Sumi, Ushio and their friends all start thinking about graduation, Ushio's grandmother is injured and asks for Ushio to come and take care of her, potentially splitting her and Sumi apart just as they are starting to have something to stay together for. Ushio's brother reunites with an old flame and their grandmother puts pressure on them to marry.

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http://okazu.yuricon.com/2015/03/10/yuri-manga-whispered-words-volume-3-english/