Peringo,
I find my self in a very similar situation, but with a twist.
After a consult with a L, and because it is a step daughter, I could only hope for visitation, not custody. If i don't take this step very soon, there is a very good posibilty that if my W wouldn't want me to see SD, she wouldn't have to. But if I file myself to make sure I would at least have visitation, she would be sure to be mad and there would be no chance to save M.

I am at a very difficult position. My D is the most precious thing in my life. But I love my W more than anything, and I would do anything to make sure we would at least have a chance.