Since you have a blackberry phone if she really wanted to and was willing to pay for the service your sister could be able to view the content of your text messages.  I have found a website that will send your text messages and voice mail to its website where if you have an account she could go online and read the texts and listen to the voice mail.  It only works for text messages if the phone is a blackberry or android.  There is a monthly charge for the service also, so she could if she really wanted to .