This will stop with normal development and maturity. Everything happens given time and love and support. If you make the child feel bad, then the only thing that will happen is more of what you don't want because of insecurity and feeling unsafe. If you make a child feel safe, then the rest will come.
The only time you should feel that this is inappropriate is if the parent is crossing some line that is unhealthy, and this doesn't sound like an unhealthy relationship. It just sounds like the child needs more attention and more time to feel safe.
Is there a reason you or your wife have a problem with sitting with the child until they fall asleep? I think it's sweet. You look at it as a special bonding time. Too soon, you are going to be wishing you had those little moments again.
Also - think of it this way... the closer the child feels to you emotionally - and that they can come to you and you will spend that time with them, the more they are apt to come to you when they get older when they want advice or have a problem...
Don't shut them out now when they are young... when because you want them to come to you, they won't because you didn't have time for them when they were little...
Blessings.
I always find this interesting. We spend all this time and energy getting our children to sleep alone only for them to grow up and have to get used to sleeping with someone else.
I personally don't see a problem with this. My son was like that when he was younger and we just worked with him in a loving and supportive manner to build confidence and security (much the way your wife is doing with your step-daughter) and he has been sleeping through the night on his own without any problems for years now.
But this obviously isn't working for you. If it's strictly because you are concerned for your step-daughter's well-being and development, there's no need to worry and you can breathe a sigh of relief and be supportive of your wife and step-daugher. But if it's something else--if it's really about you (control issues, feeling left out, etc.), then that's something you need to figure out.