Gosh, not a sentence in and I found a problem.

Twilight looked through the crack in the door to see that 'Cadence' was with her brother in there.

SETUP! You need setup, damn it! It's the start of the story! Set the scene, set the mood, set the damn table!

Shining kept getting a headache so 'Cadence' used her green magic to make it go away. But when Twilight saw this, she had two options to choose from. Option one was to run away and confront her friends about this or option two being that she confronts 'Cadence' and her brother in there.

So she chose the second option and opened the door to the room where this imposter and her brother was in.

Cadance* (I screwed this up too when I started writing, can still be seen in some early-ish chapters of Dreams of Forever)
Also, SHOW, DON'T TELL! An omnipotent narration is BORING! Give it from Twilight's perspective to give it depth and to play off her emotions!

'Cadence'

The quotes are unnecessary. Twilight doesn't know Cadance is a fake (see also: A Canterlot Wedding). I know you want to skip right to it, but it breaks the entire narrative when she has the same information as us the readers (or the narrator).

...

OOC (more than acceptable), repetitive, a train-wreck in terms of pacing, an awful sex scene, lacking description throughout the whole story, but not a bad premise.

You were wondering why you got all those downvotes? Well... It's because you need to REALLY work on the story. It needs to be torched and started over. Go read other stories to get an idea what it's supposed to look like. A good story contains description, emotion, evocative action, and makes sense! YES, even nonsensical porn makes sense in some ways!

I say start over from the top. Go read other stories to see how it is done then come back. I would say go check the 'Writing Guide' on the website, but it's... not the best. Still, the sections from pacing onward can be helpful: https://www.fimfiction.net/writing-guide#Pacing

Don't give up writing because you receive negative feedback. Learn from it, try to understand why, and change how you write (and edit) to improve the quality of your stories. Put in the effort and I know you can get better!