Refund or *****

I sent money to my sister and she never recieved money because the number that my sister was registered to is using by her and someone else.. How can the bank allow 2 people to use 2 numbers and i can't get my money back. I went to the bank no solutions.. No help and its alot R1500 because the system can show that the if number is duplicate when you apply for a card and its not the first time we are sending money to her account..