The issue here isn't about "steps", it's about Dragonstitcher being taken for granted by her DH and his sons.
Their own mother, and DS, both feel they should be learning to be more independent & responsible given their ages - and so they bloody well should be ! Their dad is letting them off the hook and encouraging dependence by placing DS in an almost impossible position. He is effectively emotionally blackmailing her by saying that if she doesn't do these things for his adult sons, she is discriminating against them.
We are not talking about an ordinary level of give and take which most families operate by .... we are talking about a sustained, regular expectation that DS will wait hand and foot on the males of the family.
Following what has been said in several previous threads about "welcoming" stepkids, I know that my parents would always have welcomed me back into their home when I was 20 (or any age) and that welcome would have included a meal if they felt so inclined, and if they had the food in. I would never however, have expected it (not even a jacket potato) as calling in without prior notice carries with it a risk that your visit might not be convenient in all sorts of ways. Of course, they would always offer a drink, biscuits etc. If I had taken to calling round every night just as they were eating expecting to be fed too, then I am sure something would soon have been said ..... just as I will to my own kids should that situation ever arise for me. It's simply manners ..... and yes, manners should apply to your own family too !
The fact DS has a low income complicates this issue but it's ridiculous (again) Xenia for you to suggest they should get better paid jobs. Did you say that tongue in cheek or do you really believe there are hoardes of people struggling by on little money who haven't thought and/or looked into doing just that ? In any case, even if DS and her DH were millionaires, that is NOT the point here. Uninvited, unexpected guests - and that includes children - may not be convenient all the time for all sorts of reasons ..... this goes back to having respect and consideration for others ....
.... Gordon Ramsay (and several other well-of people, but he's the one I can specifically quote) has said that just because his children have a rich parent, they are not going to be given an easy ride. He still wants them to appreciate the value of money and to that end, gives them £2 a week pocket money .... any more has to be earnt, though of course in his case affordability isn't the issue. Quite right too ...... in DS's case, if she did go out and magically get one of these myriad "better paid jobs" which are apparently there for the picking in Xenia-world, does that then mean that she should then dismiss any attempt at basic good manners ?