Hrrmmm... @lightingcloud I get where you're coming from, but I can't agree. Sweetness and Lightning is absolutely sweet and fluffy. Perhaps even a tad more sweet and fluffy than Danchigai occasionally.
However, it occasionally dips on to the heavy side with the whole single father bit. For that reason I find Danchigai more soothing and the over all tone lighter.
Too helpful siblings can enable social anxiety disorder, to the point of selective mutism. Satsuki isn't fortunately there, but she most certainly is some steps into that direction. Being able to rely on Uzuki so much allows her to do it.