Only God knows your sister's heart, and whether or not she honestly gave her life to Christ when she was young, and a remnant of faith still remains in her. No Christian is perfect, of course -- but some day her true relationship with Christ will be revealed. The Bible warns, "Everything is uncovered and laid bare before the eyes of him to whom we must give account" (Hebrews 4:13).
Nevertheless, you're right to be concerned about her behavior, because one of the signs of a true Christian is a changed life. In fact, if there is no indication that a person wants to follow Christ, it's a sign that they may not be saved. The Bible compares a person like this to a corpse -- physically present, but lifeless. It says, "As the body without the spirit is dead, so faith without deeds is dead" (James 2:26).
What can you do? First, pray for her. Pray that God will convict her of her godless ways, and will convince her of her need for Christ's forgiveness and new life. Then ask God to give you an opportunity to confront her about the way she's living -- not harshly or judgmentally, but with love and compassion. Let her know you care for her, and don't want her to miss being with Christ (and with you) in heaven.
Most of all, ask God to make you a witness by the way you live. She may argue with what you say, but she can't argue with the reality of a life that reflects Christ's joy and peace.