I have found myself with stepchildren also, but have been in the relationship for 3 years. It is great but its also difficult, and the older they get the more difficult it gets because you cant just say 'no dear we were just having a tickle fight' and expect them to accept it!

It took me a lot of adjusting, and i still am,there basically is no privacy all i can say is make the most of it after bedtime, get a bathroom lock to put on the inside of your door (allthough never leave it locked obviously for safety reasons) and be aware that if they hear you and their too young to know whats going on then no harm done and if their not they will turn their music up and give you a hard stare in the morning! but who hasnt been through that as a kid! luckily ours sleep like logs.
bottom line, you can never schedule time and expect it to go your way so when you do find that there is a few hours of peace dont let it go to waste! it gets easyer with time trust me.