Brother Ambrose is Prior Aymer's assistant and a monk at his priory. He seems kind of whiny, and no one gives him much respect. When Ambrose comes to Torquilstone for help when Prior Aymer gets kidnapped by Robin Hood, Reginald Front-de-Boeuf and the other Normans don't pay him any mind. Clearly he doesn't have much influence.
Giles de Mauleverer is the current Norman lord of Middleham. He stole the title from the Saxon family of Ulfgar of Middleham. He is also Prior Aymer's brother, which is how we hear about him – he never actually appears in person in the novel, but the existence of Giles de Mauleverer proves that Prior Aymer is well-connected in Norman high society. No wonder Prior Aymer has access to so much money and social influence, even from inside his monastery.