The DSSs (14 and 16) spend half the time at our house and half at their mother's. While I am very happy to ensure their sheets, towels and pyjamas are washed and changed (on the days they are not here) I think it is beyond the call of duty to actually go into their gorilla enclosure bedroom and pick up filthy clothing off the floor and clean clothing from the laundry basket and wash it for them. I think that they are of an age where they should be taking responsibility for washing their own t-shirts, underwear and jeans and I also think that this might make them think a bit before generating quite so much to wash.
The DSSs' mother, however, thinks it is the role of adults, not children, to do household chores and does all their washing for them.
The outcome, unsurprisingly, is that the DSSs take all their laundry back to their mother's to be washed 
.
We have tried explaining to their mother that we think it is time that the DSSs had some small domestic responsibilities (they do no other chores and don't clean their own rooms). She won't have any of it and sends us incendiary emails.
Quite apart from anything else, we are at washing/drying capacity during the week and would need to buy another washing machine if we were to launder clothes for the DSSs during the week when they are at school, as their mother requests.
Can anyone think of a really good reasoned argument to get the message across to the DSSs' mother?