Beautiful girls! The one thing about this is the flash, it is very strong and creates a harsh shadow. Have you tried natural light by getting them up close to a window. North light coming through a window is excellent and even with other windows, with sheers to diffuse the light makes for great portraits. I've even gone into my garage with the door up and shot with that light.
For example,
here's one of my son with the window camera right.
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Lori Putman flickr
~No one can drive us crazy unless we give them the keys
~~Life isn't about waiting for the storm to pass, it's about learning to dance in the rain!
7D | 300L f/4 IS | 135L | 35L | 100/2.0 | 50/1.4
430 EX, 580 EX II Speedlites