CSA want me to pay my daughter for my son who is no longer in education

I have paid CSA for my son over the last 9 years , my ex wife died in June this year , my son is 17 , so payments stopped as I was the surviving parent , , I found out my ex stopped getting child benefit in 2014 , but CSA made me pay up to this year , I have now overpaid by £11,000 , my daughter who is 25 has made a new claim for her brother saying she is the carer for him as they still live in the same house , he has started on a NC course and now department of work and pensions are forcing me to pay my Daughter every month for my son , he wasn’t in any education from 2014 as he was sent to a remand school , I have being arguing my case to them saying I am not legally bound to make any payments to my daughter , I am the legal father , any help and advice would be greatly received as they are now going to arrest my wages.