Each co-op has it's own regulations. It is best to find out what the "official policy" is in your particular building without giving out actual information. Then find out what the "unofficial " policy is. You can find that out in the bylaws or just casually asking someone in the building or management office. Remember to just be asking without giving them any details.
You have to be aware that most housing developments will allow temporary "visits" from family. They do not want to hear that people are using your place till they find another place to live. You are allowed to have a room-mate without question. You can have family members and friends visit without question in most buildings. In some buildings they do not allow more than an overnight visit, however I do not believe that would be the case with your building. It is best to find out what your particular building does, and then frame your story around it. Too much information to co-op members is always not that good of an idea.
If your parents move in and there is no problem, remember that at any time, someone may say something so try and keep the information to yourself.