No, because if they siblings jointly own the property or items, then each has  legal right to the proper or items--it is not theft if you are an owner. However, if one is taking more than his/her "share" and/or taking jointly owned items without mutual consent, the other siblings could sue him or her and seek a court order directing him or her to replace or repay what he or she took; as part of such a court proceeding, if the siblings can't decide among themselves on what to do with the property, the court may also order the property to be distributed among the siblings and/or that it all be sold and the proceeds distributed.