Hi, I'd like to help you with your problem.
I am sorry to hear that you experienced this with your sister. It was very smart of you to take your sister for help. Whenever you have someone threatening themselves or someone else like that, getting help is the wisest move.
It is common for alcohol users to develop a different personality while under the influence of alcohol. Alcohol, as you probably already know, is a depressant. When someone uses alcohol, it depresses reactions as well as mood. Someone who is normally fine while sober can suddenly become tearful, moody and suicidal while using alcohol. It is not uncommon that the user also does not recall their actions while drinking.
However, in your sister's situation, it appears that she may have an underlying psychological issue going on here. The fact that she acts out so violently and becomes not only suicidal but homicidal and also has such a drastic change in personality while using is a concern. Oftentimes, someone with mental health issues will use alcohol and it causes an extreme reaction and change in personality. It can make the symptoms they already have much worse. Although misuse of alcohol can cause cognitive dysfunction and other damage to a person, the same is true in reverse. Those with psychological issues can use alcohol and end up with symptoms that are overwhelming and frightening.
It is difficult to say exactly what your sister is dealing with. It is very good that she has an upcoming appointment with a psychiatrist to find out the underlying problem. It can range anywhere from simple anxiety and depression to organic brain disorder, psychosis or a personality disorder. Schizophrenia is also another possibility, however, it may sometimes be confused with other mental health disorders because the use of alcohol confuses the symptoms and makes the person harder to diagnose. Sometimes the person may also have two or more possible diagnosis and therefore, a thorough psychological exam needs to be performed to get an accurate diagnosis so treatment can help.
Once your sister sees the doctor and a diagnosis is made, she may also want to find a recovery program for alcohol users. You mentioned that you thought she might not be an alcohol abuser so she may be an alcohol user. This is someone who doesn't use on a regular basis but when she does, she drinks to excess. However, that still means she needs help, especially given her reactions when she does drink and the fact that drinking alcohol may be dangerous if she ends up on psych medication.
If your sister is not willing to go for help, you and your family may want to follow through yourselves to gain support and have a place to turn if the behavior continues. Two great places to start are http://www.nami.org/ mental health assistance and http://www.al-anon.alateen.org/ for families dealing with an alcohol user or abuser.
A couple of books that may help you are Sober Siblings: How to Help Your Alcoholic Brother or Sister-and Not Lose Yourself by Patricia Olsen and M.D. Petros Levounis M.D. and When Someone You Love Has a Mental Illness by Rebecca Woolis. They are available on Amazon.com or your local library may be able to get them for you.
Keep in mind you are doing what you can to help. Your support is important, even if she cannot acknowledge it at this point. In the meanwhile, take some time to care for yourself and don't allow yourself to become over stressed. If you feel you need to talk with someone, try a counselor. If you attend church, a pastor might also be a good option. Rely on friends and family to help as well. You will be in better condition to help your sister if you are doing ok yourself.
I hope this has helped you,