Nested Window Family Issue

[signalbass]

So I've created a nested window family by inserting a window component into a window family. I am trying to add exterior casing to the window family, so that as you change the nested window size the trim stays aligned with the outside edge of the nested window. However, the nested windows are already constrained to an editable stud pocket between the windows, so when I try to constrain the casing to the edge of the nested window I get overconstrained errors...any thoughts?