The maximum penalty for petty theft is up to $1,000 fine and up to 6 months in the county jail. Will that happen to you? No. You would probably allowed to go into some kind of diversion program, meaning that you take a class on theft. You might have a fine and some community service. You would be placed on informal probation. At the end of your probation, if you had no violations, you could return to court, enter a "not guilty" plea and be free from the consequences of the conviction.
It sounds like you are trying to decide whether to just plead no contest like your sister and go on with your life, or fight it. If your sister will come to trial and testify that she stole this merchandise on her own, that she did not tell you, that you did not help her conceal it, that you didn't even see her do it but elected to say nothing - then the DA has a pretty lousy case.
A conviction for petty theft could have consequences you couldn't imagine, like denial of entry into certain professions or denial of some student aid. If it is as you say, you need to consider fighting it all the way.
When you get to court, an attorney will be appointed to represent you if you cannot afford to hire one. Do not make any decisions without the advice of an attorney who has read the police report, the loss prevention report, has seen the surveillance tape, etc.
Each case is different and the perspetives of the lawyesr who look at it will also be different.
I am not gooing to comment on likely outcomes of the case or a DA's view might be. That was not your question.
When you go to court. Admit only to your name and ask the court for an attorney. You will have a brief conversation in which you will receive some suggestions about time and setting the case for the nesxt hearing.
The above is not intended as legal advice. The response does not constitute the creation of an attorney client relationship as this forum does not provide for a confidential communication.
If you plead not guilty, you will be represented by either an attorney of your choosing (private counsel, a lawyer that you hire) or the court will appoint the Public Defender to your case. At your first appearance, if you have a public defender you will meet him or her at that time and briefly go over the facts. Your attorney (hired or PD) will let you know if the DA has made an "offer" on your case. A good attorney will explain whatever offer was made and will talk to you about some of the longer term implications of accepting ("pleading no contest" or "pleading guilty") will have other parts of your life, such as on immigration status, if that is an issue for you. It is important to consider, as other attorneys have pointed out, that a plea may also be a bar to entering some professions. A PD is not likely to be able to review that with you. You will then decide what you want to do (and make sure that it is what you want to do and not what someone else is telling you to do). The judge will call your case and you'll go in front of the judge with your attorney. If you accept the DA's offer you will be informed that it is the same as a conviction. If you plead not guilty, you and your lawyer will begin preparation of your case, will look to see if any pretrial motions are appropriate, or if you should prepare for trial. During preparation, negotiation with the District Attorney's Office will continue and you'll be notified of any new offers.
If you plead not guilty and they find you guilty - this could only happen after a trial. You will have a jurt trial in fromt of twelve people. Should they convict you, in most counties you are sentenced that day for misdemeanors and are then given a date to either turn yourself in to the county jail or show proof of enrollment in an alternative sentencing program, like the Alternative Work Program where you can "serve your sentence" by doing tasks for the county - sometimes cleaning offices, sometimes picking up trash. If you are convicted and plan to do AWP, apply as soon as possible as you must be accepted into the program prior to your turn-in date.
I hope this answered your question. We sometimes forget that court isn't a common experience because we are there so often. It does help to know what to expect.