You should let her know about the intensity of sin done by her. She had done really such kind of act that will scarred the sweet relation of brother and sister. Her parent should know the gloomy character of her daughter. Therefore you should unmask her reality in front of her parent and relatives. This will surely be the real punish to her.
Talking with other people is not a bad thing, but it is bad in such conditions when denied confess to do so. She hides herself in such activities shows her cheap character.
If you forgive her now than, she will always ask it again and again it future. Last will never come in to her context. So strengthen yourself and let the world know about her sin. It will surely relax your mind