@Robomorons: She’s not blushing because of THAT. She’s blushing because she’s embarrassed.
 
And yeah, I can imagine RD and Rarity doing this thanks to Applejack’s advice on how to be a better big sister.
 
“Sweetie Belle darling, hold still. I’m almost done.”
 
“Why do I need these dumb floats?”
 
“Applejack saw you three doing dangerous stunts. We’re only concerned for your safety.”
 
“But you signed me up for those swimming lessons when I was five. I think I know how to-”
 
“Nonsense!”

Sweetie Belle, no. She’s your sister.

“Try not to get too wet darling,we don’t want your mane and fur to get all soaked now.”  
“But Rarity,getting wet is the whole point of swimming.”  
“Well just try not to get TOO wet okay.”  
“Uh…okay I’ll try….”  
“Have fun.”