My sister tried to commit suicide...Please Help..Thanks?

Hello.. my sister tried to commit suicide 2 days ago.She is 15 and has been depressed for the last 5 years now. For these last five years she hasnt been able to live her life at all.She never goes outside because she is afraid to.The worst thing is she cant talk to people besides my family.She freezes when people say anything to her.She has no friends because of this.I'm 18 and am the eldest in the family.I have 2 other sisters aged 12 and 17.This whole ordeal has broken our hearts.My Dad exspecially.She was always my dads favourite.She took an overdose on my dads Xanax tablets. She took over 50 altogether within 2 days.She kept walking around that sunday when she wasnt asleep( which was basically the whole day) falling around the place and being very dozy and talking wierd.I thought she was looking for attention first.When we did find out what she did, we rang the ambulance and she was brought to hospital.She was in there for 2 days and now she is out. I dont feel she should be out.One of the reasons she tried to kill herself was because school was around the corner and she was getting her results from this huge exam. I was talking to her today and she told me the main reason she tried to kill herself was because she lost her faith in God.I quote her sayng "Sarah you dont know how hard that is to feel like your faith is gone". Along with the over 50 Xanax tablets she took, she also drank 2 cans of beer. When me and my sister asked her what made her think of the cans she said because Alcohol would speed up eveything. When i asked her about what made her think of the zanix she said "Heath Ledger took them" with a wideface expression.Like this whole thing meant nothing. Im really scared. She's only 15! She's had such a hard life already.She's stunning,intelligent and the most kindhearted person you will ever meet.I love her so much. My Dad ,Mom and 2 sisters are devastated.She even said today that she cant promise she wont try to do it again.But when i started crying hysterically she then promised she would never do it agan. I dont know what to do. I'm scared she will try and do it again.She is getting concelling soon by the way.What should i do.I feel like my head is going to blow.My family are in bits.Please help..Thank You