I really don’t know, it’s weird…
My theory is that maybe she was born when they were struggling financially and she naturally associated her with their struggle and that the little sister came around a stable time so she directed all her love towards her. And with the accident she might consider again that her eldest daughter was the root cause of them having to sell their house and moving because she didn’t protect her sister.
Just a theory tho ┑( ̄Д  ̄)┍
That hyo jo guy, he got everything in his life, including her (MC), but still wants more? What does he want? Does he feel anxious because she still has family to rely on? So he thinks that she's not completely his? He's got brain damage. A mental. Or maybe she doesn't want to marry him because her family warned her, and that's why he killed them and said it's love? My poor brain.