You're facing a very hard time right now dear, but we're hear for you.

It sounds like you and your sister both have been through a lot. Thank God you have each other. Maybe you should let her know that in case she doesn't already, that you need her just as much as she needs you.
I just want to say right off the bat, don't answer or even consider these questions until and unless you feel ready to deal with them. But if you are, may I ask what psychological and situational issues have been plaguing your sister? Also, do you know (or have a guess) for what reason she took all those sleeping pills then went driving?
Maybe it is time for you to seek guidance from a mental health professional about your sister if she won't go to them herself. I hope she's at least found some (partially) effective medication.
Oh God. I'm sorry, I was going to say a whole bunch more, but it's 4:00am and it's been a strange and emotional night for me, and I think my brain just shut down. I have to get some sleep, but I'll drop by again tomorrow. I'm glad your sister is alright though. Don't worry, this will be worked out somehow.