To me, I've always look at Rainbow and Scootaloo as if they were offical sisters rather than honorary sisters.

There are some people (Especially Cough-Cough DarkMoonAnimation...) who see Rainbow and Scootaloo as mother and daughter, however, I don't think that it could work. To me, I don't think that Rainbow Dash (Even though I love her along with Scootaloo) is really mother material.

Personally, I think that Windy Whistles is a MUCH better mother to Scootaloo than Rainbow and especially Cough-Cough Mane Allgood...

I think that it would have been wonderful if Bow Hothoof and Windy Whistles had adopted Scootaloo and made her their second daughter as well as Rainbow's official little sister.