Who is responsible for creating the relationship between a stepparent and the child? The real parent.
You, as the real parent, are the most responsible one in terms of helping facilitate that relationship.
As much as you want to let that stepparent create their own relationship with the child, it’s important to remember that that’s your child. How that child feels and what that child experiences is more important than anything. It’s your job, as their parent, to make sure that they’re okay.
No matter what kind of parenting style the stepparent has (hopefully it’s super awesome and your kids love it), it’s your job to make sure that they’re good, because they’re your kids.