Based on this image:
Applejack lay sleeping in her bed, nice and exhausted after a busy day. The fence got repaired, a nice harvest of apples were brought in, the farm critters were all nice and happy, plus she even found some time to visit her friends in town.
With her eyes closed, she giggles to herself, thinking about the little trick she pulled on Rainbow Dash. For as brash and boastful as that pony was, she still shakes like an autumn leaf at the idea of revealing their secret to their friends. Naturally, Applejack teases the subject every now and again, just to make Dash blush.
Today, her shenanigans nearly caused Dash to reveal everything. However, she took off muttering about some Wonderbolts weather storm, what have you, before her performance got too incriminating. It was more than Applejack intended, to be honest. As much as she would like to tell all of Ponyville, she does have to respect Rainbow’s feelings first. Still her friends didn’t suspect anything, so Rainbow Dash would be sure to cool down by tomorrow and everything would be hunky-dory.
’Oh, Rainbow… Y’all talk big, but yer so darn cute when yer nervous,’ Applejack thinks to herself.
A big wooshing noise comes against the side of her house making a creaking noise through the window. AJ peers one eye open at it, but pays it no mind. The old house always creaks and groans in the wind. Closing her eye again, she settles back down against her pillow.
However, a hard yank pulls the blanket off of Applejack. Surprised, she rolls over to lean up, but before she can, somepony lands on the bed on top of her, keeping her from getting up.
"Do you think this is funny?"
"Wha-what?" AJ asks, still not sure what’s going on.
"All this teasing? All those times you put me on the spot?"
"B-But… Rainbow…"
"Quiet!" Dash barks in a harsh whisper.
The surprise and the tone causes Applejack to recoil back. So many questions fly through her head. What is Dash doing here? Why is she so angry? Uncertainty in her mind, seeing her closest friend on top of her with her piercing magenta eyes, soon gives way to fear.
"You know what’ll happen. You know what they think about ponies like us!’ Dash says, still in whisper but the ire in her voice coming out very clearly. "You see the way they look at Lyra and Bon-Bon. Is that what you want?"
"D-Dash-"
"Is that what you want for us?"
"I- No!"
"Then why!? Why do you do this to me!?"
"I-I…" AJ starts. Her words get caught in her throat.
Apple Bloom quietly sneaks down the hallway. She was supposed to be asleep, but she started to hear noises and voices coming from her big sister’s room. Did somepony break in? Surely AJ could handle it, but she just wanted to be sure.
Slowly, she creaked open the door.
’Rainbow Dash?’ she thinks to herself.
"Well?" Rainbow hisses, demanding an answer.
"I just thought it… I didn’t mean anythin’ by it!" Applejack pleads.
"Tell me why!" Dash says.
Apple Bloom watches in horror. What was Dash doing here? What was Dash doing to her sister?
"I j-just… liked how nervous you would act…" Applejack admits.
Rainbow stands above her, silent.
"We’re always gettin’ into our little competitions and games," Applejack continues. "So, I guess when I can make you blush like that, I…" She gulps and licks her lips. "…win?"
Rainbow slowly shows her grit teeth as her brow furrows. AJ could only shrink back to the growing anger of the mare on top of her.
"You think it’s a game? You think we’re a game!?" she asks.
"No, Dash-!" Applejack tries to protest, but a light blue hoof over her lips silences her.
"Well then," Dash says, her frown growing into a smirk. "Seems I have the advantage tonight."
Removing her hoof, she leans down and forcibly presses her lips into AJ’s. Applejack tenses up in surprise, but allows Rainbow to continue. It was an odd comfort. She still feels her anger, but this tells her she didn’t ruin their relationship. When Dash pulls back, AJ gasps in a breath of air.
Apple Bloom’s jaw drops. Did she just see what she thinks she did?
"Rainbow…" Applejack breathes out.
"I’m still upset with what you do to me," Dash explains. "But tonight… I think it’s time to show you what it feels like." She slowly leans down and drags her muzzle up and down AJ’s neck. Applejack shivers and moans slightly at the contact. "Don’t you think that’s fair?" Dash asks.
Applejack, in a mix of confusion, fear, and growing arousal, could only nod. It was enough to show Rainbow to see.
"Well, let’s get started," Dash says, planting a kiss on AJ’s jawline. "And then we’ll see who wins tonight."
As Dash continues to kiss up to AJ’s cheek, Apple Bloom steps away from the door. Applejack? And Rainbow Dash?
’But… that’s wrong. That’s wrong, it’s wrong!’ Apple Bloom thinks to herself.
She wants to charge in. To save her sister from the sinful behavior as explained to her by their own Granny Smith. But should couldn’t. Her own sister was… Tears forming and beginning to stream down her cheeks, she could only move back to her bedroom and bury her face in her pillow.
Pulling it up over her ears, she tries to block out to noise from the next room as she quietly sobs.