It’s not always easy to blend families. Different parties might be resistant to different kinds of change, and points of view can easily clash. This is even (and sometimes especially) true when you have adult children. Odds are, they have probably missed the boat in connecting with their new step-parent in a particularly intimate way. When you’re a kid, you’re more adaptable to new presences in your life. 

You live at home and you’re subjected to the will of your parents. When you grow up, they cease to have control over you in that way, so you may not elect to spend that much time with your parent’s new significant other. That doesn’t mean you don’t approve of them, it just means that your priorities might be different. But adults don’t always understand that and can take it personally when their stepchild doesn’t want to be that close to them.