Of course you can be charged with breaking and entering. What really matters is whether or not the facts meet the charge and whether or not a prosecutor could prove the charge.
When you discovered this unauthorized party at your grandparents home, did you usher the people out and call your grandparents and tell them what happened? If you didn't, the partiers will probably say that you gave them permission to be there.
You are right, you do not need to discuss this with the police. If you are charged, you should seek a criminal defense attorney to represent you. The State can bring the charges and it has nothing to do with substituting themselves as a "victim". If the law has been breached, they can charge you, regardless of whom the victim is.
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