Some of my wife's family showed up at Thanksgiving and asked me to take their family portrait for their Christmas cards. I agreed, albeit reluctantly. Though they were happy with the result, I was very dissappointed, so much I won't show you it.
But it motivated me to learn about portrait photography and lighting. I bought a couple of books, read a lot on the net and tried again with my daughters for our Christmas cards. One of the things I learned was to start simple.
The setup was in my kitchen and shot with natural light coming in from a double wide french door to camera left. It was a high overcast day with some snow on the ground outside. I also use a white mounting board to camera right as a fill. I used a painting drop cloth as a backdrop. Shot with a 10D and 70-200 f/4 L. No flash.
Needless to say, this turned out a lot better than the first attempt.
I'd appreciate any tips or feedback.

Don't feel bad about your DOF. It seems like it would be hard to get it perfect with that shot since you've got 3 subjects all at different distances from the lens. Only thing I would have done different is a) move the camera to the right just a hair to get rid of the slight void on the left side of the exposure, or b) have the younger of the two girls move to your left to fill that same void. HTH, remember, I'm a noob too 
considering natural light it's okay
