@4thtwin,
Mom is having a tough time over this. She needs kid glove treatment without caving into her unreasonable, borderline bizarre requests. The one thing your brother needs to do is to gently decline to even consider moving those poor, sad remains. He is qualified to tell her "no" because "it means more to me to have my son at rest and resting near me, his dad."
He tells her once and gently refuses to discuss it further. And refuses to let this "controversy" go on with his contribution. No recrimination, no coulda-shoulda-wouldas.
Try not to be dragged into this any further than a stock, "I support my brother in his decisions regarding his children generally and his son in particular."
I hope you all, including mom, can derive some peace from all of this and the controversy ends with as little hurt as possible.