Final answer:
Alexander I and Nicholas I were brothers, with Nicholas succeeding Alexander after the latter's death in 1825.
Step-by-step explanation:
The relationship between Alexander I and Nicholas I was that they were brothers. Alexander I, who died in 1825, was succeeded by his younger brother Nicholas I after the Decembrist Uprising. Nicholas I then ruled from 1825 to 1855 and was known for his conservative policies and formation of the world's first modern secret police force, The Third Section.