Spock's rebelliousness and emotionalism is actually something he inherited from Sarek, not Amanda, and it was inherited in Sybok as well. This is why Sybok was inclined to embrace emotion.
Even though Sybok was banished from Vulcan, he still keeps in contact with Sarek and Amanda, comming them at least twice a month, and always regards Amanda as more than a mother than his own was, and therefore calls her Ko-Mekh
He routinely comms Spock as well, performing his duties as his protective older brother, and while Spock always seems more annoyed during Sybok's comms than anyone else's, he still calls him brother.