Help please

My sister is on standard pip. About three years ago she contacted the pip department and asked if she could have a career, she has thyroid eye disease and at the time her vision was extremely poor., She cannot remember the details but is 100% certain they okayed her request to have a carer. (She had a carer for about three years) She received a letter yesterday from the department of works and pensions stating that she did not report a change in circumstances promptly and someone got carers allowance for you from 20/03/2018.. If she doesn't respond to the letter t.hey will impose a penalty. My sister is worried sick and confused because she stated that she did report way back in 2018. Any advice or guidance will be gladly appreciated.
Thank you

Thank you Bis. She phoned them up yesterday and was initially told that they did not know why the letter was sent. I was asked to read the letter out and then told that it is probably because she did not inform them about having a carer. Firstly he said it was probably because she was not claiming the component energy she is paid £25.00 not sure what it's called. The staff member said he would get a manager to phone him. Sorry for not knowing the official terms but I hope it makes sense.