
the characters have enough nuance to decipher why she made her decision (in less than 50 chapters mind you)
i think the MC's parents ideologies not meshing with her own is another factor layering in this series of events .
If she chose the coworker it would fall in line with what the parents either wanted or done themselves (╯°Д °)╯╧╧
so I'm thinking that the Middle daughter either
likes the Oldest daughter's Aide
or manipulates the Aide for information
They most definitely in love the aide always looks sad then he looks at her or when the mc talked about the marriage.
Σ(  ̄□ ̄||) it always looked like a sneer to me