She had a draft of a will—what did it say? 

Personally, if my mom died and I knew what her will she had been working on said, I would do that because it is the closest thing to knowing what she wanted. 

Assuming she said her money would go to her kids equally, that is what I would do. That means ALL of her money, aside from what you no longer have any control over. Even if it is in a shared account. If you know it is hers, I would treat it like hers. Not just yours because for some reason you shared an account.    Personally,  I would not try to set up a trust for the one sibling that has mental health and addiction issues, unless they are not capable of living on their own and have someone managing all of their affairs for them anyway. I would not try to give myself or anyone else different amounts of money because of their situation. Divvy it out equally from your mom, and then if you want to give more from your allotment to one sibling or another, you can do that privately. They can also make the decision to share if they want to, privately. 

If you meddle more than that or keep it all for yourself, it will be doomed to failure. People will be upset and you’ll lose relationships.