|Granny didn't die and get tied to the roof, though.|
Wait, it feels like I'm missing something here. What could it be?
...oh, right. It's the paper-thin premise.
Showing up in Twilight's house for no reason, Pinkie discovers the do-nothing princess* looking over genealogical records, also for no reason.
*Seriously, in a usefulness contest with Princess Diana, Twilight might actually lose by this point. At least Di is a pretty cool fountain now.
Based on the irrefutable evidence of a smudgy section of parchment, Pinkie discovers that she is kinda sorta related to the Apple family, in much the same way Dark Helmet is related to Lone Star.
On the off chance anyone reading this hasn't already seen Spaceballs 20 times
In light of this profound revelation, Pinkie sets off with her new family to bond and find out the truth of her lineage from yet another distant relative. Along the way, they sing a song that would be new if it weren't released as an animatic a year ago and rehash wilderness trip jokes so old they were a throwback when the simpsons did them.
In the end, the Apples all learn that even though family members sometimes fight, they work through their problems together and forge a stronger bond because of it, because apparently the lesson didn't stick the last few times they learned it.
But then, in the episode's true coup de grace, a revelatory twist that no doubt gave M.Night Shymalan a fatally large erection and made Dan Brown seriously consider quitting while he's ahead and becoming a plumber, Pinkie learns from another smudged document that she might maybe kind of be an Apple but also might not.
The point the episode was trying to make was that so long as Pinkie was an Apple in her heart, whether she really was or not didn't matter at all.
My problem is this: if it doesn't matter what the answer (which we never get) could have been, why do we have an episode here? Why does it even exist?
Making the point of your story be that the motivation for the story was pointless is terrible, but the general idea could have worked if there were ever any actual stakes in whether or not Pinkie really was part of the Apple family. When the answer doesn't matter at all, the question isn't interesting.
Imagine this instead:
A wealthy branch relative of the Apple family dies childless (unlikely, but possible), and based on the smudgy genealogical document, a huge inheritance that everyone quietly assumed would go to the Ponyville Apples - which they could realy use after all the potential profits and prize money they lost in Bats - goes instead to Pinkie. She could use the money too, finally being able to get her own place to live and quit mooching off the local bakers.
Already, there are stakes in the outcome. Now we can have some emotional weight.
Rather than just being a conversational side gag in Rarity Takes Manehattan, Applejack's commitment to integrity takes center stage, thrusting her straight into a dilemma.
Remember what I said about stakes? Circumstance has forced Pinkie and AJ's happiness to necessarily come at the expense of the other's. There is no easy solution to the problem, and thus whether or not Pinkie really is an Apple is not so trivial. Now it has not only emotional, but practical weight.
When two beliefs a character holds come into conflict and impel them in different directions, we have meaningful drama.
This is only a rough idea coming off the top of my head, but the point can be applied to all story writing. There must be consequences to the characters' choices, and those consequences need to matter.
Oh. Applebloom also does an x-licious briefly in a nod to the fandom. It's really lame, but I guess it's nice they're acknowledging we're still here.