See, I could understand Sarek not mentioning Sybok since he was the product of a previous (presumably failed) marriage and then Sybok drank the Kool Aid... so I could understand why he was never mentioned until Kirk dreamed him up while he, Spock and McCoy slept around the campfire in Star Trek V.Tell that to Sybok.
But why would neither of them mention Michael? Yes I know Vulcans, etc etc but she was in Starfleet. She and Spock have being a first officer in common. She stopped Sarek from doing a genocide. She gave a speech that nobody wanted before the whole Starfleet command council.
Maybe Michael is so famous that the mere mention of her name by Spock would have induced eye rolls to the point where Bones would have had an epidemic on his hands.
#OMGSpockstopnamedroppingyourfamoussister