It is always great when 'My Little Pony: Friendship is Magic' has a scenario that sees development and dilemmas between three of the Mane 6 (Applejack, Rarity and Rainbow Dash) and their younger sisters. Throughout the show, this type of relationship has always varied, especially between Applejack and Applebloom, but there are some great episodes. The sister relationship here in "Forever Filly" is between Rarity and Sweetie Belle.
Rarity is a great character and more often than not the show did a great job with her characterisation and development, some brilliant Rarity-centric episodes here. While Applebloom is the most interesting generally of the Cutie Mark Crusaders, Sweetie Belle has had plenty of great character moments and Season 4's "For Whom the Sweetie Belle Toils" proves that she can be an interesting character. It was great to see the two together in "Forever Filly" though their relationship and dynamic has been done better in other episodes. As for the episode itself it is good but not great.
"Forever Filly" is not perfect when it comes to the dialogue. It is on occasions on the forced side, a notable example being the dog analogy which was a little too simplistic, and one does have to get past the whole Rarity hardly recognising Sweetie Belle not making sense when you think of the continuity.
Mostly the moral was fine, it was a very relatable one for anybody whose bond with their siblings is quite close, for anybody who finds change difficult (applicable to me on both counts) and a lovely one handled sincerely. It would have fared even better though if the dog analogy wasn't as simplistic or forced as it was and can see where some are coming from when they consider it muddled as not everybody in that situation reconnects and that easily. It does depend though on the person and the circumstances.
However, there is a lot in "Forever Filly" that is actually done right. The animation is vibrant and richly detailed, this aspect was great from the very beginning and one of the better uses of flash animation but this shows how much the quality has come on and become more polished. The music fits beautifully, never at odds with the action and has a wide range of emotions. The writing is far from perfect but is generally mature without being too complex, it is light on humour this time round (what there is is nice) but the emotional power is pretty huge.
The story develops the relationship and bonding between Rarity and Sweetie Belle beautifully and in a way that will resonate with many who has ever been in this scenario. Rarity has been more interesting in other episodes, but is far from out of character and it for me was easy to see where she was coming from with the emotions that come with not being used to change being understandable and hit home with me quality. Sweetie Belle is also relatable and the animators and writers do well with showing her maturity, though maybe the whole different interests thing could have been delved into more. The voice acting is on point from Tabitha St Germain and Claire Corlett.
Overall, solid if not exceptional. 7/10.