What is the name of the movie about a genius young girl (very high IQ) helped his elder brother get out of a murder case?

She committed she is guilty of the crime at the stand and showed how she did it. The brother walks free because of that and can't be charged with the same crime twice. She walked free too cause she did not commit the crime and no substantial evidence either.

Can't remember the end. Would be great if someone would know.

Sounds like this episode of CSI Las Vegas
"CSI: Crime Scene Investigation" The Unusual Suspect (TV episode 2006) - IMDb (http://www.imdb.com/title/tt0769002/)

Looking through the episodes, I found another one that has the same brother sister duo:
"CSI: Crime Scene Investigation" Goodbye and Good Luck (TV episode 2007) - IMDb (http://www.imdb.com/title/tt1114941/)