This is an interesting piece. It has a lot of story behind it, and hinted in it itself. You can't get the full import of it without following and viewing the linked video. (And then you may well regret doing so, typical Gavalanche material.) Which raises questions about what interpretation to critique when doing this. I'll go for the more innocent one, lest my fragile mental state fail once more.
First up, background. You didn't attempt a background in this, which means the two figures contrast nicely with the white, standing out and looking rather solid. Indeed I'm not sure what kind of background could even be used here, and it is probably not necessary, this being more a 'slice of time' piece, a moment captured in a drawing.
This is brilliantly colored. The colorscheme here is yellow-orange and everything in the image follows it. Everything. There are hints of red and brown, and of course the eyes, but these all work with the basic colors, not against them. Especially nice is the red ribbon, (A red apple may also have sufficed.) which of course is the color of love and reflects nicely the blushes on both characters. In total everything here has a warm, close feeling to it.
The characters themselves are very well positioned. It seems you followed the traditional 'male led' paradigm here, with AJ leaning on and towards Braeburn who is both taller and standing straighter than her. I am not sure how well this would reflect canon, but given the subject material, it fits. I am not entirely sure of Braeburn's upright posture, it seems a little... flippant perhaps, especially the raised hoof.
The two individuals form a neat triangle, showing the connection of two into one and the relationship between them, at least at this point in time.
The expressions are another thing that work well here. While both share a common theme, closeness, trepidation, uncertainty, embarrassment, AJs is perhaps the more serious, the more impacting of the two, with Braeburn's being somewhat more aloof. I'm not sure I quite 'get' Braeburn's smile, though I can see the angle it is coming from. (I've seen a few exaggerated smiles like that IRL.) His wide eyes and smile give a more overblown, tense feel to him than Aj's half closed eyes and subdued smile. Again this seems to follow the male-leading paradigm.
The eyebrows over the hat are an odd, but not clashing addition. (And common enough in cartoons, though not in MLP. We need a background pony with eyebrows. Black eyebrows.)
I shan't go on, since these are the major points and I am sure others will be able to better phrase things, but overall this is a rather well done picture that becomes slightly unsettling once you know what it is based on. (Besides, inbreeding is for the royal family only, dirty mud pony. How else could we get Blueblood?)
"Honestly, I do not care if they are cousins, I saw hundreds of people who ship Len x Rin from Kagamine (And they are siblings) and several people who hate BraeJack" Actually, Rin and Len are NOT siblings. Thye are mirrors - Rin is the genderbent version of Len, and Len is the genderbent version of Rin. Its selfcest, not incest.
"I mean, Braeburn and Applejack are just cousins, fictional ponies and aren't real. I do not support incest, but I do not care if they are cousins .. I ship them." Exactly! They're fictional! Why do people care if a pairing incest/gay/pedophile? THEY AREN'T REAL! Thank you! I actually do support incest, and I don't care that they are cousins either
Ah, I get it. Well, I used to thought before that Rin and Len were siblings, but now I know that they aren't. Thank you for the information.
About Applejack and Braeburn, YES! Thank you so much for agreeing! They're fictional characters, and everyone is allowed to ship whatever they want, well, that comment I made was old, but I actually don't hate or love incest, but I'm neutral about it, but if two persons have the same age and really do love each other, who cares, anyway?
And yes, amen to that, my dear friend. We can ship BraeJack if we want to.