@amess  
My headcanon is that RD is Scoot’s real mother, but because she’s both terrible with kids and practically jobless (hasn’t moved to Ponyville yet), she gave her up a little while after she was born. Now, Scootaloo has two adoptive parents, and Dash feels as though she doesn’t have the right to take her from her loving family. She also believes that Scootaloo would hate her if she knew the truth. Therefore, Scootaloo has no idea about their true relationship. And when Scootaloo asked RD to be her “big sister,” RD took the role so she could be close to her filly without arousing suspicion.

D’awwwwwwwwwwwwwwwwwwwwwwwwwww. XD
 
Dash secretly thinks of Scootaloo as her filly, but is too shy to really act motherly towards the real thing.
 
At least the plushie has a zero percent chance of disliking it.

Rainbow Dash has secretly wanted a Daughter or at least a little sister for a long time but not only has she been worried about her reputation she been to afraid she might mess up and somehow fail the one she loves.
 
This is my head cannon.

Scootalove by proxy.

@D50  
It’s not, creepy
 
WRONG

Scootaloo obsession with Dash: Creepy  
Dash obsession with Scootaloo: CREEPY

Looks like Dash has got a little secret of her own…

Hey Dash, go get the real thing, not a plushie. I’m sure she’d like that.