
Quoting
Mr. Knowitall
Courts often do not apply this sort of clause to stepparents, but you have very broad language and practices can vary by jurisdiction, so it's worth checking with a local family lawyer to see how the judge assigned to the case is likely to respond if the issue is raised before the court. If the lawyer believes that the judge will apply the clause to a stepparent's care, then the lawyer can also advise as to how likely it would be for the court to modify the clause such that it does not apply.
You know him better than we do, but in most cases it's worth asking the father if he'll simply agree that the clause doesn't apply to the stepparent.