Stepsister with BPD, stepmom enabling her. How do I cope?

Honestly there's only so much you can do from your end. Unfortunately, it sounds like your stepmother has consistently aided and enabled her daughter to the point of destruction. It's not fair to you or your other siblings and until she comes to terms with the fact that her daughter needs help, there isn't much you can do. Dealing with a mental disability (and an apparent drug abuse problem) is way more than you, yourself, can take on alone. If no one else is willing to step up and say it's time to stop baby-ing Hannah then the cycle will be neverending. At this point and time I'd say, "Thanks for the offer on the party, but right now I would much rather worry about celebrating than about so and so's problems." Decline politely and step back. Rehabilitation and family support is the only way this girl will conquer her issues. Good luck!