Can a parenting plan require a stepparent to attend co-parenting counselling with the biological parents?

A parenting plan or separation agreement is generally an agreement between the child's own parents, so it can't impose a binding legal obligation directly on a stepparent, who isn't a party to that agreement in the usual case. What the parents can do is agree between themselves on expectations that involve a stepparent's participation - for example, one parent agreeing to encourage or arrange for their new partner to take part in co-parenting counselling as part of a broader effort to reduce conflict - but enforcing that against the stepparent directly, if they're unwilling, generally isn't realistic through the parenting plan itself.

Where a stepparent's involvement is genuinely important to resolving co-parenting conflict, this is often better addressed practically rather than as a strict legal term: encouraging voluntary participation, and explaining why it benefits the child and the household's stability, tends to work better than trying to draft an unenforceable obligation into a formal document. If a stepparent's conduct is creating a genuine best-interests concern for the child, that can be raised in the parents' own case regardless of any counselling requirement, since the court's focus stays on the child's welfare rather than on managing the adults' relationships with each other.