Bipolar Depression

OK, I've put this in 2 parts because I'd like to separate any responses along these lines. Read my first post to get info about my relationship. This discussion is for advise as to how to deal with a bp family member of my gf.
My gf's sister is also diagnosed bipolar. Back in June, she lost her job and her apartment and moved in with my gf. She takes several prescriptions (xanax, percocet, flexiril, seroquil, and probably others). I don't know the details of each or what is prescribed in what doses, but it is my opinion that she is hooked on these. In addition, she dispenses them like candy. I also know that my gf has been taking or has taken all of the above over the last several months. She has told her sister not to tell me, and I only found out after my gf's recent suicide attempt.
In addition, her sister has been trying to come between us (gf and me) at least since she's lived im my gf's house. Examples that I've heard are verbal slams aimed at my gf, but including me. She has also said that things are better between her and her gf when I'm not around. My feeling here is that she thrives in my gf's manic episodes and that they party (going out and at home) to extremes during those times. I have gone out with both of them a couple of times during these episodes and we've had a good time, but looking back is where I make my assessment.
I will detail the latest episode in detail, as it happened this past weekend. My gf was spending the weekend with me, and we were going out for Halloween and she (gf) was working on our outfits. About an hour before we went out, her sister called me on my cell phone and told me that her car was out of oil and shot, and that my girlfriend was driving in a parking lot, hit something, and when they came out of the store, there was a 'rainbow' (leaking oil) under her car. My gf was in the shower, so her sister asked her to have my gf call her as soon as she got out. Now I relayed the above story to my gf, and she asked me if her sister was blaming her (gf) for screwing up her car. At this point, I told her that she should call her sister. She did, and her sister denied that she said to me what she said, and they got into a brief exchange over the phone before the conversation ended. Then her sister send ME a text message as follows: "Hope u got yur goat making me and my sister mad I knew not 2 say anything 2 u r u happy?" (I am quoting for effect).
OK - so this ticked me off, and it is lingering. I did not respond to this message (actually my gf responded). I am wondering how I should handle this situation.
Any help would be appreciated.

I have not read the enitre post. I will say, though, that you cannot get anyone who is addicted and bi polar to behave in any way that will make sense. You are walking around in an crazy world, and trying to make it not crazy. People who are ill are ture to their illness. ADdicted people are true to their addiction. Relationships come second, if at all. I am in my 50's. I have lived through it all. Beleive me, you cannot change these people by havbing a 'proper' reaction. You are caught in their webs. They are not bad people, but they are messed up and not in treatment. If I were you, (and I know we all do this in our own time and in our own way, and this advise is probably not going to be what you want), I would exit the insanity. I am sure you can find healthier people to hang out with. who cares, in the end, what these very troubled people think? It is called 'stinkin thinkin' in addiction recovery groups. It is called 'crazy making' in therapy circles and by doctors who treat these people. Help yourself first. They will do their own thing anwya, and you are not much more to her than a prop. I am sorry. I do feel compassion, but life is short and you have a right to be treated with respect and integrity.