Why are we still having the full amount taken if the mother is under investigation?

My husband has been paying csa for twin girls he didnt know exsisted until 4 years ago from a one night stand 19 years previous!!

The mother turned up out of the blue after 15 years demanding payments my husband gullible as he is paid the mother no questions asked?? myself and the family urged him to seek dna testing which csa refused to do as he had made numerous payments.

Anyway 4 years later the twins are now 18 and at 6th form they leave in june. Our problem is 1 of the twins moved in with us in dec 2012 for a week short of 3 months my husband was still paying csa to the mother?!she went home in the feb only to move in with her gran in the june!!

We have since learnt she had left one of the courses in 6th form and is now only doing 8 hours?? we have been told the mother is still claiming for the twin as living with her and being in 12 hours education?! she is apparently being investigated for benefit fraud?

We spoke to csa who told us it was under investigation but the csa payment was still taken out of his wages this month??