Awww sweetie I am so sorry! It sounds like she needs a target for her anger and you are handy. It isn't fair at all to you and I can imagine it is very painful. I have experienced a similar situation with my brother but unfortunately I didn't gain any wisdom from it. My brother is an adult so the situation is a bit different. The only thing I can suggest is to just let her know you will always be there for her and then, hard as it will be, just let her work through it and hope for the best. Just remember you have done the very best you could do and sometimes other people's problems are just beyond us. You should be very proud of yourself for all you have done!
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226