I believe what you are witnessing is not a slip but a full-blown relapse. I've lived through this more than once and it takes on this same pattern. She's lying to cover her addiction. That won't last very long because everyone will figure out she's not ready to maintain sobriety. I am so sorry you are anguished about this and I understand it because I've gone through it too.
I wish I could give you some remedy or magic words that would help you to make your sister realize she is trashing her life right now. But I can't. One thing I can assure you of, however: if you and your family get upset, worried, go into rescue-mode, or fly off the handle at her, she won't give a flying fig. She has her bottle and that is number one in her life. Her family is coming in a distant second. This is what it is all about with an active alcoholic.
She's dying for a drink and drinking herself to death. Literally. The only way to get some help for your hand-wringing and worrying is to keep posting here for support and get into Al-anon. Folks there have walked in your shoes and they will offer you understanding and support.
Love your sister. Just realize you can do nothing to make her stop drinking.