Yes, I am very lucky! No question there!
And, not to sound too sappy, but I /get/ what you are saying. I care about SS a lot. I can honestly say that in a way I love him. I scream my head off when he gets a hit at the game. I sit on the floor and commiserate with him when his dad is really hard on him about something. When he hated, hated, hated his summer camp last year, I drove my pregnant self over there every day to pick him up early. (I was out of work on disability) He is a wonderful kid.
But, when I see my daughter, my heart melts in a way it never has before. I can see, how if it were her, this would not seem like such a sacrifice. (I may get totally flamed for this. I'm just being honest)
However, we do need to have this work for us as a family. I think, as a step-mom, I have to be very thoughtful and deliberate about this, because, as much as possible, the fact that my feelings for SS are different than those for my daugher should affect, as little as possible, how I treat him.