Why Spock never mentioned his sister Michael Burnham ?

This question was explained at season 2 finale of Star Trek: Discovery. After discovery crew went to the future to protect sphere data from control. Tyler, Spock, Pike, and Number One were debriefed by Starfleet after this incident. During the debriefing During the Spock suggests to his Starfleet interrogator that in order to protect the safety of Starfleet's past and future that all files on Discovery’s mission, crew, and spore drive should be sealed, and that any Starfleet Officer with knowledge of these events should keep it under the penalty of treason. This helps explain why Spock never mentions Michael throughout his Starfleet career.