First Seroquel is given for many reason now days from sleep to drug withdraw that have nothing to do with mental illness in 25mg to 100mg dos. .I take it for my bipolar to control the times when im manic and finding the right dos. was hard ive been on between 100mg. to 800mg . right now I take 200mg. with 200mg. as needed.Now as far as i know most people like your sister take 600mg. to 800mg sometimes higher .I have friends that told me that it works for them but it wont get rid of the voice but just lessen them so they can live a normal life .My own opinion is I love Sq. it just took time to find the right dos.My doc. use to tell me this all the hated hearing it but its true .THERE ARE NO MAGIC PILLS . I wish there was! But it takes time & a lot of hard work to live with a mental illness.

Good luck with your sister