A family guard is hired by a queen and assigned to her own prince. Affinity with and loyalty to the prince may vary, especially if the queen has multiple kids and plays favorites. An example of high loyalty but low affinity is Sale-Sale's guard captain. He works for Swinko-Swinko and has Sale-Sale's best interests at heart through her, but Sale-Sale is happy to get rid of him for a few hours.
A spy guard is part of the royal hierarchy system, assigned by a higher queen to the prince of a lower queen. Zero loyalty to the prince, spies at best and assassins at worst.
Sorry if this doesn't clear things up.