I am a step-mom too and I have a son that is fixing to graduate from high school. In our home my husband and I have always had rules that have to be followed. He is a good young man with the same problems of a messy room and at times not motivated to do anything. We have had to talk to him and asked him what is he going to do with his life. It seems when you ask questions and get them to start thinking they have more ambition to do something and getting them interested in some kind of activity that they like or might be willing to try. I know they are young adults but have you really sat down to talk to your step kids and really find out what might be going through their minds? Are you really listening to what they are telling you? Is their real mother involved in their lives? Have you ever had a good realtionship with them from the start? That is the key word for beginners. Just like all the responses that you have gotten I agree,you have to make them accountable for something. So, maybe they can help with your business, helping your parents by taking food,cleaning, etc. I agree you should pick a day and have datenight with your husband. He may never complain but you need to make time for you and him. As well, you need to have family day or night for you,husband and the kids. Life may be better if you took the time out to set these 2 days out for your family. Because when you said "I Do" you inherited the whole package. It's not like you didn't know he had kids. They too need to have respect for you as well.
There is home health out there that maybe your parents qualify for so you can get some help and that can at least give you a couple of hours to make time for yourself,kids and husband or make time to run your business better. FYI: Lipet Home Care, Inc. is a home health-###-###-####. You should involve your whole family in the business because this will give them the incentive that you too care about them as well because your willing to let them help you out. Maybe this might be the motivator that will help them to clean up their room, help out with shores,business, parents,go back to college,etc. Who knows this might be what they need but nobody has ever approached them or asked them. If none of this helps maybe you should seek family counseling from a professional out there just google it to get someone that works with families.
Sorry hopes this helps.