You need to consult with a criminal attorney. There are sometimes ways to accomplish what you ask, and sometimes not.
[In accordance with the Avvo community guidelines, this communication does not constitute "legal advice", nor does it form an attorney-client relationship.]
There are ways to petition the court to review the NCO put in place post-conviction for possible rescission of the order. The judge will generally consider all the facts and circumstances of the case, as well as the victim's input and the treatment (if any) the defendant has been engaged in since the time of the incident. Consulting with a local attorney familiar with the process in Domestic Violence cases could assist you through the process which can be somewhat confusing. I'd be happy to provide a free consultation to discuss the matter more if you would like.
Jeff Holmes - Attorney at Law - www.vancouverdefense.com - 360.975.9288. Disclaimer: This answer does not constitute legal advice. This information is based on general principles of law, as well as my general experience that may or may not relate to your specific situation. This information is not meant to take the place of actually consulting an Attorney in your jurisdiction. If you would like legal advice, I would recommend consulting an attorney in your locale.
Mr. Holmes is correct, and also apears to be a local attorney who practices close to where you reside. Knowledge of local practice is helpful, particularly when a reviewing judge has enormous discretion about what to do. A local attorney may know what 'works' with a particular judge. Ultimately, it is your sister's responsibility about what to do, and she has to be prepared that your father can also come to court and voice his concerns. Since she is now an adult is the eyes of the law, the Court/judge must listen to her position about contact with the boyfriend. As Mr. Holmes states, the decision to rescind or modify the NCO is going to up to a judge who will consider a lot of different information. Your sister needs to talk to an attorney.