The incident took place in Mosta Bakora village of the district when the duo, going to a bank at Manch, were resting under a tree, revenue officer of the area Nath Singh said.
Apparently a nest of wasps in the tree, under which they were resting, was disturbed by an animal and the angry insects assaulted them, Singh said.
While Kamala Devi (70) died on the spot, her grandson Roshan (15), died a few hours later, he said.
"We have appealed to the district magistrate to help the family as it is deep in grief after the sudden death of two members," Singh said.