My SD13 leaves her stuff all over the house also, clothes, dishes, hair products, straightener etc.
I refuse to clean up her mess. So when I am cleaning up I take everything that is hers and I put it outside her bedroom door. I mean everything. If she leaves a water glass out it goes outside her door and not in the kitchen. She can take it to the kitchen herself. That way her dad sees it sitting there and he tells her to pick it up. That way there is no confrontation and no arguing. Its in front of her door. She knows it belongs to her. She knows she has to put it away.

P.S. As far as him not eating what you cook...then he starves. His problem not yours. Explain to your kids that their step brother is choosing not to eat and that there will be consequences for him later, just as there would be consequences for them if they didn't eat.

I look at it as you need to instill in YOUR children the proper values so that THEY can choose to do the right things. you won't be able to shield them from every obnoxious, slobby kid in school, just like you can't shield them from the obnoxious, slobby kid at home. So teach them to do better.

It's the only way I can survive with our BS4. SD15 is a nasty, lazy slob but is the Perfect Precious Princess in our house (well in her and DH's eyes)DH (supposedly) tells her to pick up her room, throw her garbage in the trash etc. but nothing ever changes. But I have taught BS4 better, I have taught him to pick up his things, I have taught him to brush his teeth, etc. I have taught him that these things are NOT an OPTION. He is already at FOUR able to see the difference between how SD acts and how he is EXPECTED to act. BS4 will actually tell SD15 to go and clean her "nasty room", or to brush her "nasty teeth". It's quite hilarious. Even DH laughs about it.

While it sucks to have the double standard, I see it as the only way I can survive this crap without going insane

Don't worry too much about your children being led astray by SS. YOU are the authority figure in your child's life and that far outweighs the influence of another older child. Plus the age difference between your bio and your stepson is so large that your children will see SS as a kidult, not as a peer and won't really expect consistent treatment in the way you might think, or emulate him, as long as you are consistently enforcing YOUR rules for YOUR bios. One rule for him, another for them will seem quite normal to them if they grow up with it.

On the dinner issue - as another poster says, the real consequence of refusing to eat dinner is being hungry. If he refuses to eat the meal then politely suggest that he get down from the table if he doesn't want to eat. It will pull the rug out from under him if he is attempting to assert control or cause annoyance. He either has to get down, or eat something. If he continues to moan or complain, just keep repeating pleasantly 'sorry you don't like the meal, but I won't be cooking again this evening so you should just get down from the table if you don't want to eat'. If your DH then wants to prepare something separate for him that's his issue.

When SS was younger he had a very bad habit of leaving his stuff all over. I would only pick up items that would endanger the baby or toddler and they usually went into the garbage. When the baby chewed and slobbered on a book he borrowed from a friend and SS had to replace it with his own money he was pissed but when the toddler broke his mp3 player he was livid and never left ANYTHING out again. It was an expensive lesson he had to learn twice but it eventually stuck.