have i committed a crime?

Although I do not practice law in the state of Michigan, here are my initial impressions.  First, you have stated that the police have been contacted by your sister's girlfriend, but you do not state whether the police have spoken to you yet.  If you have not yet spoken to the police, my advice is to refuse to speak to them without an attorney present.  The police may already have probable cause to arrest you for a destruction of property-type of charge, based solely upon your girlfriend's sister's potential statement.  However, if you speak to the police you will risk incriminating yourself with respect to this charge, as well as a number of other more serious charges that may also arise out of the incident, such as threatening, assault, etc.  Therefore, I suggest that you consult with and/or retain an attorney for the investigatory phase of this incident, and defer any and all contact by the investigating officers to that attorney in the interest of minimizing or eliminating your criminal liability arising out of this incident.  Remember, from a financial perspective, it is usually much cheaper to retain an attorney for an investigation (that does not result in an arrest) compared to retaining an attorney to defend you with respect to crimes that you have already been charged with.

You have committed a crime--at a minimum, it will be some type of vandalism or damage of (expensive) personal property. Depending on the circumstances, such as if your girlfriend or sister were intimidated or threatened by your action, you may also have committed a crime by threatening them as a well.

You therefore *could* be arrested. Whether you will be depends on many factors: how it was reported to the police, whether you have a prior record causing them to take your actions more seriously, the guidelines of the local police department, even the personality of the officer(s) who took the call. Also, how you act if the police speak to you will affect what they do from there. The police might arrest you, might give a summons to appear, might simply talk to you. You should be prepared for the possibility of arrest.