Some excerpts from the tail end of ‘Sleepless In Ponyville’:
 
Scootaloo: [sighs] I’m so, so sorry, Rainbow Dash! I just wanted you to hang out with me and see how cool I was so you’d take me under your wing, teach me everything you know, and become like my big sister!
 
Rainbow Dash: Sure! I mean, I got over it because I realized pretty quick that if there was such thing as a Headless Horse, I could totally take it on. So, you’re looking for somepony to take you under their wing, huh?  
Scootaloo: Mm-hmm.  
Rainbow Dash: Yeah, I might be up for something like that.
 
[waterfalls rushing]  
Sweetie Belle: I call sister teams! Last herd to make it to the falls is a moldy carrot!  
Rarity: Ugh, if you insist. [giggles] It is so on!  
Rainbow Dash: They think they can beat the two of us?  
Scootaloo: [wings fluttering]
 
So no, they do not retroactively become biologically related, but clearly Rainbow Dash becomes Scootaloo’s adoptive big sister during this episode. Now, in the image above, Rainbow Dash has turned Scootaloo out, which is problematic, as, even setting aside their relationship being sororal in nature, Scootaloo is a child!

@Lillian Sandra  
It all really depends on how you view it really, they portray in multiple episodes that Scootaloo and Rainbow Dash have the chemistry that siblings would have with each other if they get along, and not fight 24/7, in Sleepless in Ponyville, Scootaloo even says herself that Rainbow Dash is like a sister to her.

@Lillian Sandra  
They’re sisters. Blood has nothing to do with it.

They’re not sisters. Remember, Hasbro said they are just friends, not sisters. This is not true about them being sisters.

Cute, but also mildly suggestive. I kinda love it.

@monolyth  

What are we gonna do on the bed?

cute af