You need to try and get her to admit she has a problem. She sounds very depressed. When I was suffering with depression I thought along the very same lines. I did not see the point in living, thought everyone would be better off without me and that if I disappeared no one would notice. You can never see beyond.
Its like being in a crowded room full of people and no matter how hard and loud you scream - no one looks up to help you or comfort you. It's a very lonely illness and caused by a chemical imbalance of the brain. It causes alsorts of mood swings and irrational behaviour - but is uncontrolable without the correct help and guidance. You & your family will not be able to help her because you will not be able to understand. Someone like me who has been through will understand to a certain degree, but the only way we can help her is if she wants to help herself. There are many different types of depression and you have to be very delecate in the things you say to her.
Click on the link below and have a read - it is very informative and may help you to understand a little better.
http://hcd2.bupa.co.uk/fact_sheets/html/depression.html
Although I used to say I did not like being me and I everyone was better off without me - I never actually meant I was going to hurt myself and become suicidal. If I was going to do anything it was just disappear - but some how my logical thinking remained and I stayed put- finally realised I had a problem when I was suffering from panic attacks - went to the doc - who diagnosed me with depression.
Sometimes isaying things like "I wish I was dead" is a way of communicating how messed up and unhappy you are.
I don't think at this stage you need to be thinking about getting her committed, but I would recommend you try and get her to see a Doctor and then go from there.