Semi-connected thoughts time, go!
So, after rewatching the episode, I think this is one of the episodes that appreciates on repeat viewings. It is not our usually lighthearted show, this thing is serious all the way down. It's way more serious than Sisterhooves, which had plenty of silly comedy moments to break up the heavier stuff. Even the funny bits we do get are only a gateway to tragedy (SB's "big girl" makeup, anyone?).
Like, this bit is something that only makes sense in full context.

Rarity has just rushed in with the dresses, and Sweetie Belle's sense of good taste is ringing. She knows these dresses are fab and way better than the ones she did. Sweetie was expecting alterations, not a re-do from scratch. It brings back memories of all of the times that she thinks Rarity showed her up—true or not.

But she decides to put that thought away. "No, Rarity wasn't doing this to show me up, she just wanted to help. She's not lying." Doubt is clouding her judgment. The show isn't ruined, and they are pretty fab, so whatever, she'll roll with it.
Sweetie Belle is fishing for compliments at the after party, and it's clear that her parts of the show might not have been the best, so the other ponies spare her feelings by lying through omission. They thought that giving Sweetie Belle any kind of compliment would have been fine, so they complimented the dresses—exactly what she didn't want to hear.
Aside from sibling relationships, it's also a cautionary tale about accepting the flaws and trying to improve in one's own creative process, even though it's not at the forefront. SB is too emotionally invested in her own work to recognize its flaws, while the older, more mature ponies don't give her a straight answer in response to her prodding for reviews. She blames the success of the show (or rather, lack of compliments towards her) on Rarity instead of stepping back and realizing that hey, this was her first stab and that it wasn't exactly High Art. Accepting that Rarity was not out to get her also forces her to actually think about why those dresses were getting complements instead of her own work.
It's not exactly front and center, and a whole episode could be done on a moral dealing with this, but for a secondary moral it's good. The girls gave lip service to being gracious, modest, and classy, but in the very next scene the CMCs (and especially Sweetie Belle) are all looking for compliments and strutting around like they own the joint. Being gracious and modest is about doing things for their own sake, because they are good on their own merits, not because you want to impress other people. It's a pattern of behavior that goes way back for Sweetie Belle—hence the fifth birthday party. She thinks being like Rarity means being a diva, being showy, and getting compliments, but what being like her sister REALLY means is giving more of yourself than you expect in return.
As far as

goes...

This is the look of "take your medicine, kid." Throughout the whole thing she's very serious, and pretty quiet. As a teacher, Luna is letting Sweetie Belle work through the problems on her own. Sure, she could have skipped to the big finale, the "this is what happens because of you," but it wouldn't have addressed the core problem of the relationship. The obvious parallels are there between Luna/SB and PC/Rarity, but Luna isn't just coming out and saying it. You could have slid PC into Rarity's role and Luna as SB in the fifth birthday party (just change the plot to something about royal duties) and gotten the exact same emotions, but it would have been a bit redundant and not as relevant to the episode's primary characters. Luna only briefly mentions her past; it's a perfect example of a "show, don't tell" in the context of teaching. Luna could have lectured SB about the dangers of jealousy, but she was able to show everything first hand. SB learned from her own mistakes and matured a lot.