OP - I don't know enough about maintenance and Child Support etc to comment on the ins and outs. However, I would say you are being given a hard time on MN over your question.
I totally understand where you are coming from and, FWIW, I have been married to my husband for over 20 years, we have 4 children and neither of us has step-children, so I have no vested interest in this thread.
However, if my DH had a child, who had been poisoned by DM and was NC, I would be keen to know if said DM was claiming illegally. Also, if DC is earning good money and is paying DM and you are also paying DM, then i would be looking to put money directly into an account for DC rather than give DM even more money. Do you know who pays for DC clothing etc nowadays? I have a DS who is 19 - he works full time, earns good money and pays housekeep to me. Obviously we do not get child benefit for him, we feed him, he has his own bedroom, showers daily and uses a fair amount of electricity. But in the scheme of things, I really don't think he costs a huge amount extra. Therefore, I'm afraid I don't see why everyone is saying that CB and housekeeping from DS would be nowhere near enough to keep DC.
I admit we struggle, but I guess what I'm really saying is I sympathise with OP and her husband who are being given a hard time here when it is not that they begrudge dc the money but rather feel it unfair that DM seems to be claiming money she isn't entitled to, as well as taking it from DS and OP. From my understanding, OP is not wishing to fund DM lifestyle, rather than stop supporting DC.