I Mean If We're Being Honest Quirin Was Slacking On That Oath More Than The Other Two But Eh - Tumblr Posts
I like to think that when it came to following Edmund's orders, the Brotherhood can be put into three categories: positive, negative and neutral.
Hector is positive because he followed them to the letter - he did everything in his power to stop anyone from reaching the Dark Kingdom.
Adira is negative because she actively disobeyed by helping Rapunzel get to the Moonstone.
Quirin is neutral since he gave up being a knight to settle down and have a family, so he wasn't actively affecting the situation one way or another. The one exception to this was when he warned King Frederick about the dangers of uprooting the Sundrop flower.
And yet, in their own ways, they were all still upholding their oath: to protect the world from the destructive powers of the moonstone.