Is there anything I can do?

My sister, 15, is out of control (I feel)... She practically lives a secret life - I found around 60 + condom packages in her room last week and later found out that she had already lost her virginity at 14 to her 19 year-old deadbeat/homebody boyfriend... She took pictures of herself naked to send them to her various 'lovers'... and she continuously steals money from our stepsister, 12, ~ $20 this week alone ~ She also has horrible eating habits and had gained ALOT of weight she is around 60 + pounds overweight now and insists on wearing the skimpiest clothes... I love her and she's my baby sister I just don't know what to do... Any advice will be greatly appreciated!