Our seats given away to passengers of overbooked seats

My sister and I had each bought a ticket to fly to OR Tambo just to arrive at the airport to check in (still on time) and for Kulula attendants to inform us that 5minutes prior to guests' arrival they give their tickets away to the overbooked seats. They were unable and unwilling to check us in seeing that they had already given our seats away to other passengers. We lost our fligt costs and were reimbursed with a shy R133.00 voucher. We were so disappointed and had to repurchase another two flight tickets from Fly Safair who accommodated us kindly and we were late to our sister's wedding in Pretoria.