This feels kind of unfulfilling for Mio. He was actually the one who worked the most in this series, three times more than anyone else. He was also utterly selfless. You might say the conclusion made him happy, but it's because he's happy when Shizuku is happy. However, the problem there is that he needs to take care of that airhead big sister, who would be hopeless without him. So, while he's happy, that happiness doesn't really have anything to do with him. He's not her father, not even a big brother. He's a several years younger little brother, and it's tragic his happiness is derived from succeeding in taking care of the big sister, not from his own life. I know Shizuku has no wicked thoughts and is innocent, but she's totally taking him for granted. Furthermore, not in a single scene I can remember could you say she was worried about him or how his life is going on.
If this series had continued a bit longer, I have a feeling Saki would have actually paid attention to Mio's situation. That would have been jolly good. But, alas, no.