I don't know your full situation, of course, but if possible, I hope you can maintain contact with your sister and do whatever you can to help her and encourage her to change. The Bible says, "A friend loves at all times, and a brother (or sister) is born for adversity" (Proverbs 17:17).
After all, the time may well come when your sister reaches the end of her rope and begins to realize that she's been traveling the wrong path in life. The Bible warns that "the way of the unfaithful is hard" (Proverbs 13:15) and every day we see evidence of this truth. Satan promises happiness and escape from all life's problems but his promises are lies.
This is what the prodigal son in Jesus' parable discovered and once he did, he decided to return home to his father (see Luke 15:11-32). You should pray that your sister will discover this truth before it is too late for if she doesn't, she could end up destroying her life. And if she does wake up, she may need you to help lead her home.
Most of all, pray that your sister will come to Christ and give her life to Him. Only He can take away her rebelliousness and guilt, and only He can give her the peace and joy she seeks. Pray too for your parents, for this is a very difficult time for them, and they also need Christ's help and wisdom.