Rii and Miya have been close friends in Hello! Project from the very begining they were chosen to H!P Kids. As Miya is one and a half year older than Rii you could see their relationship as big sisters caring little sister.
I myself see their friendship more equal and mutual. In Rii’s and Miya’s friendship we see how two different kind of persons match perfectly together. It’s that yin/yang thing again. When Rii is impulsive and dreamer Miya is more calm and feet on the ground person. And they go so well together. As an imouto Rii respects Miya and you can see it when she looks at Miya in certain situations.
And when these two have fun together you can only laugh with them and wonder how crazy they can go when there is no camera capturing them.