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.