In Hawaii, Rainbow Dash would be known as Scootaloo's "hanai" big sister. Not really her sister by blood or adoption, but a beloved mentor who takes on the role. So yes, Dashie really is a big sister in that way, and it's obvious she loves being a sister.
I bet when Scoot gets her cutiemark she gonna brush it off like its nothing after embracing the waves of emotion from the night Dash took her under her wing. I wanna get hugs from Dash & Scoot after seeing this beautiful image.