good stuff. You're keeping the proportions correct. Hips and shoulder at about the same width. Try to pitch your darks more and shade in the midtone and leave out the highlights. HOpefully someone can help you with the armpit since I"m struggling with that also.
It is hard to see if you are using sufficient construction methodology... but it looks like you could stand to take a little more time building up the underlying structure of your figures and faces before making the leap to shading/rendering. What appears to be happening, since I do the same, is that you have a photo, and you are just trying to recreate its surface appearance and you are skipping trying to understand what makes it look the way it does. It is a hard habit to get into, though, but worth it.
good use of value to show the skin and the clothes. YOu should focus on improving your line qualities and pushing the values more. YOu have hiccups in your lines and they're quite dark and bold compare to the rest of the pic while your shading is soft and doesn't indicate the form. Pay attention to the shadow shapes, especially cast shadows since those give the most indication of the forms casting the shadows and the forms the shadows are casting on.
hope this makes sense.