Wow, what a lot to go through. As hard as this is for you, I can't imagine how difficult it is for the two teens to leave their friends and their home, move across the country, and move in with a father they barely know as well as a stepmom and 2 little ones. I'm a stepmom of 2 girls so I know the challenges.
On the plus side, the girls are old enough to voice their opinions and wishes. I think it's nice to tell them what your schedule is but you should also make it clear that you know YOUR schedule will change to accommodate them as well. I'd tell them that you'll all work together and take it one day at a time. Set up some meetings with the school about how to successfully integrate new kids who have gone through a major life change, may have a sense of loss as well as fears of being accepted in a new school, fears of changes in curriculum, etc. I'd also look into a family counselor so the kids can have support and you can work together to establish bonds and a sense of security. Be aware that teens often don't share a lot - especially with people they don't know (like you and their father as well as a counselor).
I wouldn't do too much in their room(s) right now - let them know that you'll take them out to help them purchase some things to make the rooms their own. Hopefully they will bring some of their own things with them. But a new home can be a metaphor for the new family - you're all starting fresh in a way.
Let them sit with you and their father (or just him) to develop some family rules - you'll have the final say of course, but giving them a chance to have input is a major step in giving them some control over their lives. They are in a position of feeling they have ZERO control, so you will have a better get-acquainted period if they have some investment in setting up guidelines.
Let your husband be more of the parent than you - I know that is hard for you but he is their father. I don't know what the story is with their mother, but it may be harder for them to accept you right off the bat if they are conflicted about their mother, missing her, being angry at her, etc. It's not your job to take care of everyone else - everyone is going to have needs here, including you. I really think professional help is going to be key so you understand what they've been through, can voice what you are feeling, and have a sounding board as well as objective advice.
You might read up on family meetings and how to organize/run them.