Oh, I am so sorry to hear about your sister. This is a very difficult time for you and your family.
Your sister has bipolar disorder? If this is the first indication of her disorder then there is no way for anyone to have known or helped her. A manic or depressive episode can come out of the blue. Many people that suffer from this disorder have a traumatic episode and then the disorder is diagnosed.
I don't know that your sister will have any concrete answer for you if you ask her why she attempted to end her life. Deep depression is very hard for many people to understand if they have never felt that way. Depression of this magnitude goes way beyond just the blues. People that suffer from this type of depression have such profound sadness and despair. Life seems to hopeless and they feel powerless and helpless to see beyond the despair. This type of profound depression zaps the energy from you. You are unable to perform the simplest activities of daily living. Getting out of bed, showering, grooming and eating take monumental effort. There seems to be no way to end the deep sadness and pain. This sadness and pain may not have any specific reason or cause. When the chemical imbalance in the brain occurs, neurotransmitters are not released in adequate levels. This results in deep depression and excessive bouts of energy. It is not something that is easy to see in many people, especially when it is a new onset of the disorder and symptoms are not recognized as part of a larger problem like mental illness.
The most help you can be to your sister is to support her. Try not to be judgmental about her behavior. You are seeing the worst side of depression. Bipolar disorder has two spectrums to it. Mania or hypomania (a milder form of mania) is the other side of bipolar disorder. You may have seen some of the moods and behaviors that are associated with mania and not recognized them as manic behavior. Educate yourself. The university of Google is a good place to start. There is sooooo much information out there. When you go to see your sister ask the nurses and the doctor to help you and your family get information about bipolar disorder. They can get you literature to read, refer you to support groups and assist you with finding the answers to any questions. I can not encourage you enough to get educated about this disorder. Knowledge is power. Your parents will need to empower themselves to be able to help your sister. You will, too.
I don't know that you could have helped your sister if she did not have a diagnosis of bipolar disorder prior to her attempt to commit suicide. This may have been her first episode with the disorder. Your sister will need medication to restore normal balance of the chemicals in her brain. She will be given medication while in the hospital and remain on meds, probably for the rest of her life, unless there is some breakthrough in science and a cure is found. Bipolar disorder is treatable and can be managed but not cured. Many people live productive, happy lives with bipolar disorder. When you see your sister just show her love and support. Let her tell you things as she feels comfortable talking about it. That may be right away or it may be never. She may not have any answers herself. Just being there and showing her you are not angry with her will be tremendously helpful. Let her know she is loved, cherished, and valued - no matter what.
Keep coming back to this forum. Many of the participants here have bipolar disorder themselves. They can tell you about life with this disorder first hand.
Again, I want to say how sorry I am that your sister is sick. She will recover and manage her illness. You can get through this! Get support and education so you know about her illness and can support her.
My thoughts and prayers are with you, your sister and your family. Let us know how all of you are doing. Peace.