I think i figured out the windmill incident

Ok so hear me out

What if the windmill was a model. A toy. So the sister took a picture of the two kids holding the toy. Marvin, being the loving and compassionate person he is, threw it into the street. Lena went to get it, but was hit in a hit and run. Marvin, being maybe 12 or 13, dragged the child into a shallow grave.

The second picture was marvin standing over the unmarked grave. He didnt know she took it.