Youngest daughter being touched inappropriately

My youngest daughter who Is 4. I fostered her when she was a baby. I now adopted her without their father in the picture. I also have a son and a new spouse. She told me that my oldest daughter touched her inappropriately. My oldest daughter is 7 years old. He also told me that her sister told her not to tell anyone. I just learned that my oldest daughter was abused. I'm in shock. I never saw it coming. I'm heartbroken and confused. What should I do?