What to do if my son just got a ticket for harrassment?

If you want to try keep this off his record or get it off his record, then it would help if you could obtain an attorney for him.  Considering it's a ticket, there may be some options that are easier to invoke than, let's say, with a felony offense.  The first is to see if the prosecutor will accept an affidavit of non-proseuction from his sister.  Essentially, she tells the court this is an isolated incident, related more to bad medication, and that she doesn't think the charges should go forward in the interest of justice.  The second option is to see if the jurisdiction offers some type of diversion program.  Not all do offer this, but those that do will require a defendant to complete a class or counseling, pay a fine, and complete community service--- then once they have met the program requirements, they dismiss the case outright.  It's like an informal version of a probation, but without the plea of guilty.  There may be other options available in your area that a criminal defense attorney can direct you to, but these are two fairly common options that you should look into to keep the charges off his record.

I suggest that the best way to resolve the situation with your son is for you and he to retain a criminal defense attorney to see how you may resolve this dispute to the satisfaction of the district attorney's office, the victim (sister) and the court.

Assault and battery as well as domestic violence are serious issues in this country. Possibly the court may place your son on court probation where he is required to obtain scheduled medical treatment, do community service work, go to counseling, attend classes regarding violence and if successful, the charge may be dismissed.

The above is solely an option to try and resolve the matter and not a sure result.