hi Smh13 - this is madness. I have 2 DS and 2DSSs. All of them put all of their own clothes away. They are all aged between 12 & 7. They also all put their clothes in the wash. DP/I wash them. DSS1 often leaves piles on his bedroom floor. We do not move them. They do not get washed unless they go in the laundry basket. He knows that.
They also all strip their beds every other weekend so that I can wash it all while they are at their other parents' house. DSS1 used to hate doing this, but when he refused to do it, he just had to sleep in dirty sheets. The next time, he did it, and now just does it automatically without being asked.
We no longer iron the kids' clothes because they ram them in drawers and we said we would no longer do it because there is no point. If they could prove they put them away well, we'd iron. The 12, 10 and 9 yr old can iron anyway.
They also ALL have to clear away from the table, and clear away after playing etc. They ALL lose pocket money/electronic device time for rudeness etc.
The point is it doesn't matter whether they are step, or bio, they are all learning to be part of a family, and to live with other people. That means taking the responsibility along with the benefits.
I think your real issue is if your DP and his ex listen to this excuse. It is a pants excuse (hahahaha!!). Please tell me your DP isn't taking this excuse seriously..???
There may be different rules in your house and their mum's house. we have this issue from time to time, with DSs about their dad's, and DSS's about their mum's. My mantra is "his/her house, his/her rules. My and DP's house, our rules". And our rule is you all put away your own clothes.