Depending on your available space, the macro would be a great portrait lens but it will be a tight head shot. That is especially true if using it on your T3i. If your limited on space, I would use the 50mm f/1.4 and stop it down to f/4 or f/5.6. You should be fine with either of those two lens combinations. I would bounce your flash as others have suggested or position your subject by a large window that doesn't have direct sunlight. That usually produces pretty soft light. Go to your local craft store and buy some white foam core and use it as a reflector (or anything you have that is big and white). You could even have her hold it underneath. You wouldn't even need to use flash if you have something like that. See this video by . She uses natural light for almost all of her photos. This one has a big window camera left and a big white reflector type thing camera right. It's a little risque for head shots, but you get the idea about using natural light and using a reflector. A google search for indoor natural light portraits will yield a bunch or tutorials also.

Good luck and don't forget to post your results, we'd love to see them.

Jayson