I actually kind of thought they were like the older roses when I saw them (mostly Ritsu)

is there some kind of joke going on with these k-on maria-sama pics?

yeah, they're look alike ^_^
yui must be yumi... out

Yumi's hair is almost always done up with short twintails, she never wears a hairband like ritsu has on. The choice of which characters to imitate doesn't really make much sense, ritsu -> Torii Eriko, Mio -> Ogasawa Sachiko and Tsumugi like Toudou Shimakou. Each character is in a different year and aren't related as sœurs; doesn't really seem to be any kind of theme behind the picture.