Something like, she had a dark first experience with sexuality and has made it her life's mission to never let that happen to Cirno. The way she looks in post #1864514; with a pained, empty, thousand-yard stare into the distance; seems to imply she is flashing back to something that hurts to think about while she scolds Rumia.
Look also at how Dai looks in Hammer's older art as compared to now -- she went from being a slightly endowed preteen to a fully grown young lady. Symbolic? Coincidental? I don't know, it's just fun to think about.
