Marriage politics is a way of gaining power in the
government of any sort
of
monarchy.
In it, an individual or family will try to have their daughers or sisters
marry eligible bachelors within the ruling family. In this way, the
male children of such a mating will be royalty and thus in line to assume
the throne.
His mother, loyal to those who wanted to gain power, can then influence
her son to act more favorably on her interests, moving people into strategic
positions within the government to gain even more power.
Entire governments have been slowly taken over this way, perhaps most
notably the Fujiwara family gaining power over the emperorship of Japan
in exactly this fashion during feudal times.