I drew the lines a little bit away from where the photo shows the edges, so that people could see that the image itself draws those lines. The edge of the lining can be seen, and the stairstep part is above the line made by the edge of the lining.
It’s something that makes a distinct square - not triangular - edge, and something that has enough width to it that the light reflects off of it. This isn’t a drape.
Maybe it’s a pillow. Most caskets have the head elevated at least a little. But if that’s a pillow they sure didn’t have her head on it. And if it’s a pillow that sticks above the box then it should have shown in the other images. Whatever it was, it should have shown in the other images.