miccisue is 100% right. The only way your brother will change is if he wants to change. Though it's easier said than done, you must realize it's in no way your fault, and, you should not be the one feeling guilty.
If he's threatening suicide, see about having him admitted to a hospital. Also, if he's threatening suicide, he's likely depressed and may need medication (if he's not already on it), and/or therapy. However, your parents are truly the ones responsible for him if he cannot be responsible for himself.
I don't know your family situation, but, is it possible to have a sit down with your parents and address your concerns?