Complex sentence: The uncle is placed in prison and is to be executed, but his Republican nephew feels that it would be unjust-and helps him escape, whereupon the nephew is court-martialled under his own decree.
Split into simpler sentences: The uncle is placed in prison and is to be executed, but his Republican nephew feels that it would be unjust-and helps him escape. Whereupon the nephew is court-martialled under his own decree.