(credits to the artists and Photographer)
One of my favorite pictures I think this is much what Sin would look like in Lamia form, cept she wouldnt have the patterns and scales over her humanoid torso and slightly bigger breasts.
Here is roughly what Sin would look like in her Naga form, cept she doesnt have the cobra like hood, nor does she have the wings, and she would be a tad big longer in the tail, not to mention she doesnt have nipples when in naga form.. and her scale coloring is the same color as the snake seen in the picture below.
This is what Sin would look like if she were in snake form, the scale coloring is exactly what Sin's would be as she is an anaconda descentant, so the scale coloring on her Lamia and naga forms would also be the same as the snake form scale coloring.. of course she is about three to four times larger then the snake in the picture.