Interesting theory -- except this: Are you referring to the photos in the link below? They show his sister hugging him after the scattering of the ashes, but not "leaping into his arms with joy"; and Michelle is nowhere in evidence.
The emotional moment Barack Obama scattered his grandmother's ashes
That’s not the picture I saw. I’m looking for it, but it’s probably been scrubbed. This was perhaps later in the day, near their cars, taken from a long distance by a photographer they were unaware was shooting the pictures. I saw it earlier this spring, and sure wish I’d bookmarked it or something. I’ll post the link here if I find it.