Tough position you're in...but I'm not too clear on what is going on in your situation. You said you "are still headed for divorce" and then go on to say, "She wants you to be her husband, however." I also do not know the age of the children involved, which has a big impact on your role, as does the role of their biological father. However, based on the information that you did give, here is my input:

1. You are NOT their dad...do not try to replace him in their lives.
2. You ARE their step-father...be their role model & mentor when you can!
3. Be supportive to their mother, and openly/honsetly express your opinion when differences do occur.
4. Set appropriate limits on their behavior & set clear, consistent boundaries!
5. Discuss this issue & your concerns with your wife, and DEMAND what she expects your role to be! You cannot be excluded from the decision making process if the two of you are to be married.
6. VERY IMPORTANTLY: Realize that the children should be your wife's priority over you, yet you deserve "your" time as well!
7. MORE IMPORTANTLY: DO NOT fall into the trap of ever blaming the children for ANY marital discord that may ensue. You may feel they are to blame, but never reveal those feelings to the children! To be successful in a blended family, you and your wife MUST be able to discuss the issues, expectations, disciplinary response, etc. And you MUST be able to reach a mutual compromise. Hope this helps, a little, coming from someone who has been there done that!!! Good luck and keep us posted!!!

__________________
I wish all the ladies were pies on the shelf...and I was the baker because I'd eat them all myself.
I wish all the ladies were potholes in the road...and I was a dumptruck because I'd fill them with my load.