Watching someone we love destroy themselves is no picnic, and watching others we love be hurt by their addiction is hard too.
What helps me to let go is to do what I can...speak openly and honestly about what I see and be willing to support their actions to get better, and then to let go and say a prayer handing them over to God to take care of today.
You've done what you can, talking with them about this, and now it's time to let the chips fall where they may. Taking a front row seat watching and trying to direct the outcome, only makes us crazy and doesn't change what they choose to do.
You're a terrific daughter, they are lucky to have you. Just keep taking care of yourself and by example you may be the light they see at the end of the tunnel.
Hugs