I offered another solution in an edit.
Otherwise, give your brother a title in your land.
OR give the child a title in your land.
They will remain loyal to whichever title is highest, and if both are equally high, whichever title came first.
Try and make the kid a duke in YOUR kingdom first.
Or make your brother a duke in your kingdom and hope your brother dies before the duchess dies.
...
Or do what i'd do and make SURE he dies first.
If they don't want to be independent, they wont be. Search for them plotting, or keep an eye on it and wait for them to declare independence, then intervene as an ally.





