Should I attempt to sue my local post office?

My sister used to live with me. When she moved out a year ago, she had her mail forwarded to her new home in another state. All of her mail is being forwarded to her new address. We have contacted our local post office more times than I can count and they cannot solve the issue. Meanwhile, my wife's credit score has plummeted due to late bills because she isn't getting them. The post office said that it's because their names are similar and the computer can't tell the difference. I don't know how else to get them to fix/resolve this.