Can the csa take ALL of your wages?

my sisters husband has been told by the csa on his next pay day the csa are taking ALL of his �2200 wages and everyone after that until his arrears are gone.His ex got the csa onto him around 8 years ago he managed to get them off his case by paying her direct BUT she hasnt told the csa that now he has �12k arrears.and before anyone says it YES he should have got proof of payment but silly sod didnt anyway ive been in a similiar position with the csa as myself and my ex split acrimonisly and the csa told me they can take upto 40% of my earnings!!!so surely this should be the case in his instance?? my sister has a decent job but the csa have no info on her income so its not as if they are basing this on the fact they could survive on my sisters wage and also they have a 1yr old baby together so where do they stand??? Any help would be greatly appreciated