@kikyichi
For the most part it seems like a tale of two sisters who have struggled to cope with the environment they've been raised in and the sudden "freedom" they've received from losing their mother. It's also about how perspective is everything.
The eldest sister has been held to unreasonable standards of being the "perfect" eldest daughter, and it's had adverse affects on her mental health. Whether she realizes it or not, she's become a carbon copy of her mother, despite remarking that they're "free" from her mother after her passing. She's jealous of her younger sister due to her perceived freedom and not having to put up with their mother's incessant demands, and often feels inferior due to not being as smart as the youngest. The eldest is also jealous of the youngest's perceived lack of responsibility, as the eldest has always had to be the "responsible" one of the family, giving up on their own personal expression to be of use to the family.
The youngest on the other hand has had it equally rough, realizing early on that the eldest was jealous of her (what with the eldest hiding the aunt's dress.) While the eldest had grown up in a stifled environment due to their mother, the youngest grew up in the same circumstances, as the eldest unknowingly turned into a copy of their mother, and often belittled the youngest with offhand comments here and there. The youngest hates it when people compare her to the eldest, due to the fact that the eldest is just a perpetuation of toxicity from their mother. She realizes this early, and also realizes she has the freedom to escape, which is why she's opting to go for far away colleges she can live in at rather than commuting to them.
Both of them have grown up in toxic suffocating circumstances, and it shows in word choice and their actions. Both's personal growth has been stifled completely, though for wildly different yet strangely similar reasons.