I can understand where police would want to duck this as to a domestic debate.
Your state may have an unlawful detainer type law that fits.
Personally I think that parents, brother and GF probably need to retain 1 attorney of behalf of all to seek an unlawful detainer, return, damages etc from the galpal. Essentially only the property owner has standing as to its being detained by others. The victims need to be assertive! Unless we are talking about expensive furniture there is unlikley to be much money in this for an attorney so you all may need to be creative to get the right talent in place. Hint--if the 4 victims can somehow jawbone 4 theft complaints into place the gal may not know the police have zero zeal to follow up. Hey, 4 theft actions places her at serious risk--she might be inclined to see the light--CAUTION to extort a civil remedy under threat of a criminal charge is just that, a form of extortion itself--don't go there--its probably done all the time by lawyers but they know how to use words more skillfully that you do!
But as a tenant of a furnished rental unit if you as a co tenant strip the place of furniture, bedding etc I may have additional damages against you because you have rendered the place such that I cannot readily sleep or eat there.
If there is an oral lease for rent the parents can selectively go after galpal for unpaid rent