I'm with everyone else - you are not being unreasonable, at all.
I live with 3 step children, we have them 50% of the time. They don't do a huge amount of cleaning, but they definitely tidy up after themselves. i.e. make own beds, put pyjamas away, unpack lunch boxes from school bags, put shoes and coats on hooks, tidy own bedrooms, etc. They might seem like minor things but I got hugely pssed off when I first moved in and they did nothing* at all.
I just used to get cross (on the inside about it) but then realised that i was pointless, so sat down with my OH and we came up with a list of things that were reasonable for for them to do i.e. the things above, and then things like everyone clears the table, and helps set it etc.
These things have now become routine, and they do it (on the whole) without too much prompting. They know the rules, and they are simple: they clean up after themselves, and help others when they need help too.
As two are under 10, we also used a star chart which worked wonders, and tied it to their pocket money (the youngest two get about 20p so it's not expensive!). I know you probably shouldn't bribe children, but the 50p pack of star stickers from tesco and the tiny amount of pocket money is money well spent as it helped encourage them, and now I don't need to bribe them as they just do it.
The key thing is for your OH to back you up so you don't get the inevitable thing of them running off to whinge at their dad about how are you asking them to clear up.
Failing that, tell your OH to clean up after they have gone, or pay for a cleaner for Monday mornings when the step children have left... His children, his responsibility.