Do you think eldest would agree to rehab? I know you've said she tries to quit but if she's anything like I was her "quit attempts" might actually be part of an ongoing drinking cycle that includes some days off. And it's not that I was being dishonest... I actually thought I could and would make the necessary changes each time I tried to stop. But (as I think it does for most of us) it took some pretty significant warning signs and disasters to wake me up from that delusion.
I would say try to avoid making it into a situation where she feels you're the one forcing her into it.. as that only gives her AV more ammunition. But it certainly can't hurt to offer. I can't imagine what it would be like as a parent, knowing you're that last remaining safety net. But if the safety net is preventing her from having to figure out another way.. she's only getting herself further and further into danger.
I also think it's a great idea to stay in sober living til you have 6 months if possible. As long as you keep protecting your non-drinker status as the first priority, the rest will continue falling into place.