Yuu face blush analysis:
Kuzuki: Least Yuu face blush, unsurprising.
Niko: Most Yuu face blush, again unsurprising as she is clearly the most pushed sister.
Miwa: Close to Niko level of Yuu face blush, just slightly under. Compliment word was "Sparkling" which seems very noncommittal compared to "Special" and "Lovely".
Tinfoil hat time, Miwa Dark Horse Theory:
As stated earlier, Niko is the most pushed sister, however I believe the author is setting up a bait and switch. Miwa has definetly gotten the second most attention through out the manga, but her arcs have contributed more to the on going story then Nikos have, even introducing an on going character in Sakura, Miwa's friend/rival. While "Sparkling" isn't as bombastic as "Special" or "Lovely" the author probably did this on purpose, ie "Yuu can say those things more easily to Niko and Kuzuki, but because of how he feels about Miwa he could not.". The noncommital "Sparkling" paired with the very close level of Yuu blushing that Miwa received compared to Niko confirms the Miwa Dark Horse Theory to me.
WE BELIEVE IN MIWA SUPREMACY