My theory is that the oldest sister knew what the real motive behind their mother agenda in marrying the fl to crown prince.
There's something feel iffy about the mother marrying the fl to the crown prince despite the mother looking down to the empress and all her daughter got married happily even the divorced 2nd daughter is chilling
I think she's just jealous of Tatiana. Remember when Regina came back -- there was a flashback before Anessia married her husband. Anessia was in love with another man and Regina offered to help them run away together, but instead she caved to her mother's demands and married the noble. Tatiana refused to be that girl and fought against her mother instead and now is living happily with Kainel. Anessia is probably jealous and regretting not seeking her real happiness.
her eldest sister, anessia is her name i think? there’s something off about her, not sure what it is though ╮( ̄▽ ̄)╭