It's likely she was keeping the family afloat. Fred was "a young master!" (Abusing authority and position) and FMC was the hated one.
Sister might have been a bystander for the abuse. So once she was framed she quickly gave her an out
Guessing her "disgust" is how much their mother plays "delusional" when it suits her. Since FMC is hated, she's the bad one, since Fred is "loved" she enables
Even if OG FMC had the means to fight back she STILL would have been the bad guy to that woman. I'm sure Fred has skeletons in the closet they both are aware of which explains the disgust better with her acting like he was an innocent angel.....despite sending him to kin slay his little sister he was "pretty eager to do!"