Why do family members turn on one another?

I had a younger sister about 15 years ago left her family destitute. Our mother died from cancer. We tried living together only for financial reasons. However, one day, she never returned home. She turned her back on us, basically, calling me every name imaginable. She took some of my belongings and promised to give me $20 for the items. The $20 was to turn on our heat for the winter. We owed money to the gas company and they said if we paid the back bill they would do it for us. We found out that she stopped paying the heating bill and her part of rent. I went to her friend's home to get the money. She lied about the money, but threatened to call the police on me for taking her shoes. I left the shoes and walked through a foot of snow and drove home barefoot. We didn't have heat on when it was 10 degrees outside in a trailer. We basically we freezing to death. My aunt finally told us to stay with her until we got the money for the bill. A few years later, she came to us begging us to forgive her. We made amends, but I still didn't trust her. I should have listened to my inner feelings. I received a credit card in my name and my aunt's name. I only signed up for the card in my name. She asked to use the card to buy a vcr. She lied to my aunt and said that I had used the card in my aunt's name. She did this deliberately to separate my family. My aunt threatened to have me arrested. I went to the local police and showed the officer the card. I never did speak to my aunt again. She died in 2006. Later, my aunt's neighbor told me that she thinks my sister and her husband did this to declare my aunt incompetent and take her home. My sister was always jealous of me. She was jealous of my boyfriend, my art abilities. I think she suffered a nervous breakdown after my mom died. Why would a loved one change so drastically and basically stab her siblings in the back? I still care about, but I will never totally trust her again. Is it possible to make amends to my extended family? My sister at one time wanted to be a missionary. I have forgiven her for all the horrible pain she has caused. The one I feel the worst for was my poor aunt. May she rest in peace. I have a wonderful new family and extended family. I am happily married to a wonderful guy.