I'll put up some "translations" (transliterations), but generally, the point seems to be what suffix (or lack thereof) they're using. ("-san" is more formal/polite/distant, while "-chan" is familiar/casual, and lack of suffix is intimate or casual to the point of rudeness.)
I.E. Yuugumo calls everyone else "-san" except Kagerou, so that makes that particular relationship significant, while Shiratsuyu just doesn't use suffixes, which says something more about her than any of the relationships.
YuugumoYuugumo-chanFubuki-sanYuugumo-sanMutsuki-sanKagerou-chanFubukiFubuki-chanThey mutually respect each otherFriends that trust each other deeply* The concept is that, besides with her sisters, Yuugumo-neesan is a bit shy, but because of the mutual respect with Kagerou, they call each other simply by name would be nice.Get along well
Often go overboardMutsuki-chan
