Hi Edhy,
I have struggled with similar UI presentation with my customer maintenance window.

I opted to utilize the tab control. This allowed the most footprint for the window. There are seven related chunks of data that I am displaying on each customer record (notes, addresses, contacts, quotes, orders, shipments and invoices). That is a lot of data to squeeze into an itty-bitty living space. There are far more than seven related tables, however. Each tab may reference several other tables to get the data presented.
In addition, I also use a modal window when adding a note, address, or contact. The rest are view only, but a double-click on the appropriate line item (and with the proper authorization) will open a maintenance window for the object.
It is all quite fast. Quite painless to put together.
The main reason that I bring this up is the possibility that your interface will grow. For example, what if the user desired to see a list of dependents. The way you have it structured, the addition will put you into a tailspin--where do you fit another group box in? For a tab control...just add another tab and fill it in with the data that you need.
I am still working on the customer maintenance window. Always will. Users keep coming up with good ideas to implement.
HTH,
Bill