I once heard some good advice from a family counselor regarding blending families. Spend one on one time with your bio child. It works for the stepchild as well!
Divorce is a big enough change for a kid, then add a new marriage? Blended family? He/She most likely feels lost in it all. A good way to validate your relationship is do something with your kid(s) one on one through the week, or a designated time once a month. It will reassure them that their place in your heart/life will remain the same no matter what.
Not surprisingly, spending one on one time with your stepchild/(ren) can have the same effect. It has helped that my husband has one on one time with my son. They play golf. My son lived with us the first few years, and then when he hit 14 or so he did what a lot of boys do, he moved in with his dad. He left on good terms, it's a good thing for him to have that relationship with his dad. He's also involved in their church/ youth group. But when he comes to our house, my husband always sets aside a time to go play golf with him.
My stepdaughter and I have a lot more one on one time, because her dad works over the road. But when its just us on her weekend here, we will watch a movie together, or go to eat, we've even done the pedicure thing. Sometimes we just sit and talk. Of course, we've been working on this for five years, hitting bumps in the road..
But doing these things can help dissolve a lot of hard feelings, jealousies, resentments. It shows them their place in your life, helps to secure them.
A good rule of thumb is: how do I want my son's stepmom to treat him? And that's how I try to treat my stepdaughter. We didn't ask for things to turn out this way, and they sure didn't. They deserve our every effort to give them the stability they need to be successful adults, and to have healthy relationships themselves. Hope my ramblings helped someone who may be overwhelmed or discouraged. Keep the faith! God bless.