My niece is horrible

My sister is the kindest soul you would ever meet, and a very loving mother.She has three daughters: 6, 8 & 10.The youngest and oldest daughters are sweet as sunshine. But the middle daughter is a nightmare.She has tantrums over the smallest things many times per day. She explodes with foul verbal abuse. She physically attacks her sisters and grownups. She has tortured animals.My sister has tried therapy but the daughter didnt engage or else was all sweet and the psychologist was like 'your daughter is absolutely fine'.Lately my sister cries every night. She is at the end and doesnt know what to do. It is seriously destroying her whole family.Any advice? via /r/Parenting http://bit.ly/2CIbhQb