I found the picture of the young Sister Lucy (around age 15) in an online version of Frere Michele's The Whole Truth About Fatima. The picture from 1949 is on a number of foreign websites. Several of them provide a link to what I believes was Sister Lucy's convent, which is where they found the picture. Unfortunately, the link no longer works, but they also include the video I linked to above, which shows the picture hanging on the wall at the convent. Here's another picture of what appears to be Sister Lucy 2. This one is dated 1951, and was also supposedly posted on the convent website, in the link that no longer works.
Something else I've been noticing is that the chins may not be as different as I once thought. I'm beginning to think the difference is due to the angle and the added weight. In the picture below, you can see that Sister Lucy 1 has a long chin that protrudes at the bottom. If you add 50 lbs, and take another picture with her head tilted slightly upward, the chin may well look like that of Sister Lucy 2.
The smiles are different, but compare the mouth of Sister Lucy 2 to that of the biological sister I posted yesterday. They look the same. That makes me wonder if there could be another explanation. Notice how flat and short the teeth are in Sister Lucy 2. It looks like they have been filed. Also notice that the gums in both pictures above are larger on the right side (from our view), and extend further down the front right teeth. If Sister Lucy I had her front teeth shaved, the teeth and gums might look very similar to those of Sister Lucy 2. Could this also have something to do with the difference in the smile? I don't know.