KN17, do you see a therapist? i think it would be really beneficial for you to be able to talk to someone who isn't directly related to you and is removed from the situation with your family.
It sounds as though your Mum has her own issues to deal with, and you are not responsible for that. I am so sorry to hear that your Dad isnt very well. It's hard to see someone we love becoming ill.
In regards to your sister, give her time. It's only been about a month since you told her about the drugs/drink. She may be in shock and just needs some time to deal with it.

Like you said, SI isn't a good way to deal with any of this. You need to get outside help from a therapist or someone similar.
Good luck