PROBLEM:
An in-place family is visible, but when you try and edit it objects within it are not visible.
CAUSE:
A family is a container for other objects. When you create an in-place family within your project objects within that family will be put on a workset (which is not the case if you create a family as a separate file).
The family is also on a workset. Objects within the family inherit the workset of the family, until you edit the family when they revert to the workset they were created on.
So the family is on a loaded and visible workset, you can see it, but when you try and edit the objects within it, which are on a different non-visible or unloaded workset, they disappear.
SOLUTION:
Change the workset objects within the family are on to match the workset the family is on.
You will need to do this in a view that has all worksets visible, and/or load all worksets.