Since your father died intestate a Grant of Letters of Administration is required. Until such time as this is granted, no one has authority to deal with the assets since the authority is conferred by the Grant. So no, your sister should not have done this.
Unfortunately, redress will be expensive. The police will not want to know (although technically it is theft, she will more than likely say she had the understanding that it was for her to deal with). You could seek a full account and inventory from the Probate Registry, but this will not be possible until the Grant has issued.
You could seek the intervention of the Court for an account and inquiry, but this is likely to be very expensive. Your best course of action is to negotiate with your sister and get her to make a compensatory payment out of her half of the estate to compensate you for the loss of goods (if indeed they have been lost rather than just stored).