There are few guidelines for stepmothers and stepfathers about how to understand child development or to parent effectively, instantly. They jump into middle of a kid's development and try to make sense of it. Usually with little support.
Sharing a child isn't as risk free as it might seem. Sure I can appreciate that my daughter has extra people loving her, and she gets to have a different perspective in parenting, but there's a small selfish part of me that still wants to dominate all the highlights.