I just went thru these same concerns. I too, have a 5yo stepdaughter, she'll be 6 in october. As it turned out, she was THRILLED to be a big sister, this will be her first sibling from either parent. What I did is make a really big deal out of how special a big sister is, i bought her a super cute "i'm the big sister" shirt to wear to my baby shower and at the hospital and she has helped us get her brothers room ready.
One of my main concerns came when we decided to move her out of her room and make that the baby room. So before we did anything to the baby room, we had a talk with her and told her we wanted to give her a brand new "big girl" room. She loves her new room and I think that really helped with the transition.
I have made her involved in everything, from taking her to the baby shower to making sure that she was with us the weekend her brother will be born. We just keep reinforcing to her that we both love her.
The only thing I've noticed is that she has become VERY clingy with me. I don't mind it, I love her, and fortunately i recognize it for what it is and make a point to reinforce to her that this baby isn't going to take her place in my heart.
The other thing her daddy and I have been adamant about, and fortunately her mom agrees, is that we do not ever refer to them as Half brother or half sister. This is her brother, her REAL brother, and nobody better say any different. We've been very lucky in that her mom was on board with this and really supported us (she knows that when she has another child, we will support her as well). She even sent down gifts for the baby that she and my step daughter picked out together.
All and all there has been nothing negative about this experience where my stepdaughter is concerned.