Well, this is a hard situation for a mom. I would first make sure nothing has happened to your 7 year old. These thoughts, if it happened, do not just pop into a child's head. There is a chance that someone touched her or that she saw it somewhere. Casually ask about this in a supportive, non confrontational way. Then make a firm ground rule that we can touch ourselves in our rooms but we can NOT touch anyone else. Period, end of story and consequences will follow if this rule is broken. Kids at this age can be pretty rule bound. Then monitor them. Also talk to them about no touch by anyone than themselves and if anyone tries, even a family member or sister we trust, that we immediately exit the situation and tell you.
She told her not to tell because she was abused also.