"You...ain't winnin' this here little go around, Rainbow." Applejack said, feeling her consciousness and alertness levels dropping dramatically. She was so tired...the softness of the rug underneath her frame and hooves, the warmth of the fire, the dim lighting massaging her eyes, and feeling the shallow, relaxed breathing of her sister, snuggled to her side as she was...it was all ordering her to lower her head, close her eyes, and let Rainbow gloat in victory if she must. But the orange pony held her head steady, and glared back at the pegasus stubbornly. She wasn't going to loose to her; especially in something like this! She loved that Rainbow and Scoot had finally come to terms, but she and Applebloom were still the more intimate siblings, and she had to show the pegasus that first-hand. You can't just meet your surrogate sister and expect to beat-out a sisterly relationship that's been going on since the younger one was born, after all!
"Come on AJ, give it up!" Rainbow ordered, stroking Scootaloo's mane a bit. "No wings means less comfort to offer for getting your sister to fall asleep~"
"Will you cut it out with your wing-talk, alreadah?" Applejack retorted, her sleepiness leading to a raised voice from the tension and annoyance of Rainbow's intent to compete. "Body parts aren't what count; Applebloom and I have been sleepin' together on this here rug for years, ever since she was a teeny tiny filly. You just got Scoot to be your official surrogate sister less than a hour ago!"
"She's been seeing me as her idol and sister-like figure for more than a year," Rainbow said confidently, winking down at the filly in the folds of her feathered wing. "We may not have experience, but she's probably way too excited at the prospect of sleeping with her new big sis to NOT fall asleep first."
The two rivals glowered at each other, shooting daggers.
(You can probably have Scoot and AB say something along the lines of how they aren't about to fall asleep anytime soon with them arguing xD Since AJ and RD are far too oblivious)