How to frame around an egress window
The basic answer is that since this window is already set in the wall the interior framing that you are adding does not provide structural support of the wall over, under and along the sides of the window.

On the other hand you may want to consider how the interior trim and furnishings will be applied. If you intend to apply a wide casing around the window then do consider that having doubled framing around the window can be very nice for nailing into. This can also be very nice above and to the top sides if you intend to install wall mounted drapery rods or bars.