Hello Louba, welcome to the nutty wold of stepmums. 
If you have read my recent posts I have the same problem as you, in as much as my dp or my stepkids will not do their fair share, I have decided to start to change it.
This evening dss walked away from the dinner table leaving his dirty plate, glass and a lot of mess behind, I said "erm dss, you are not going to leave that there are you?", he looked at me a bit blankly, "go and put the dirty dishes in the dish washer, then you can clean up the mess you have made". He just did it without having a sulky teenage strop, he just did it without batting an eye lid,
mind you I am very tired, he knows I have been up since 6am, without much sleep all night (see previous recent posts). I had just been telling how tired I am and what my day is like and he seemed quite shocked and he told me he didn't think that he could do it. 
All this time I have just been (as I suspect you may be doing) doing it all myself and not making the stepkids clean up after them because its my home not theirs, but I also realised today that the stepkids do see my home as theirs. It was something dss said on the way home, he said "you know where we live there is that...." then he went on to describe something which is very close to my home, up until this point I had thought that he meant BM's house when he said "where we live". So I thought "well, if they do think of it as 'home' they can blooming well clean up after themselves."
I really don't know if this makes sense because I am so tired, I guess what I am trying to say is ask them to help out, you never know, they may suprise you and just do it!!!!
Now dp,......... That's another story!!!!!!!!!!