It's a good example of something that might sound simple, but actually involves a great deal of complex work to support. As it stands families are independent collections of other object types - a nice and simple system. If you can add families to families, then it transforms that in to a tree structure of families. Changing the structure of the tree (e.g. by adding/removing/moving families in families) will have complex repercussions in your project regarding which instance variables, behaviors and effects are inherited through the tree, and cause various parts of the project to become invalid, which the editor has to handle (e.g. by deleting events referencing things that are no longer applicable).
It can be done, but it's time consuming and painstaking work to do upgrades like that, and as ever we get far, far more feature requests than we can possibly do, so we have to be pretty ruthless about prioritising.