Sadly, yours is a common dilemma. A few weeks ago, I wrote an article about manipulative adult stepchildren. Here is the link:
http://www.bellaonline.com/articles/art23185.asp.At the risk of sounding like a broken record, please pay special attention to the book I recommend at the end of the article. It's called Emotional Blackmail, by Susan Forward, PhD., and it would be an EXCELLENT resource for both you and your soon-to-be husband.
I agree that we teach people how to treat us. Your fianc� has taught his children that it's okay to not only disrespect him, but also you. You can stand up for yourself to a certain degree, but he really is the one who has to recognize what is happening and then to change it. And the sooner the better!
He needs to realize that no amount of coddling and catering to these spoiled "adults" will make up for lost time. They are no longer minor children but adults who behave like kids. Others live up to our expectations of them, too. Obviously, he expects them to dish out the punishment he deserves. I would say that he has allowed the roles to switch. They�re now the parents and he�s the child. That�s the dynamic you�re witnessing when they�re blatantly disrespecting him. Talk about emasculating!
You probably aren't going to like what I'm about to say, but you should consider waiting to marry until your man has had the epiphany of what his children are doing. I'm not trying to be pessimistic, just realistic. If you think it's bad now...DOUBLE the frustration after you marry! You may think that marriage will make it better, but believe me, it won't do anything but give these "kids" a new goal in life--to destroy that marriage.
Your fianc� simply has to get his head out of the sand. He left their mother, for whatever reason, and now they're blaming you (and disrespecting him). If he allows this to continue, it will only get worse, and worse, and worse. Is that what you really want for your life and your kids?
Children (even manipulative, interfering, immature adult children) are a major reason that remarriages fail. Your soon-to-be husband needs to make you a priority in his life. If that means that he demands his adult kids act like adults around him and you, then so be it.
You can't MAKE him do the right thing, but you can control what YOU do until that time. There are many second wives out there who feel like you, and have been relegated to lower than a servant in their own home.
The sad fact is, once these "kids" succeed at taking an inch, they'll grab a mile. Many a second wife has even been excluded from having Christmas with her husband because his children refused to see him if she were there. Only you can answer if you're willing to take that chance, or not.
I'm glad that you and he are in counseling. That's a good start. Maybe it'll help your fianc� to see the light soon.