With Michael being the adoptive sister of Spock, the series has many flashbacks to their childhood and upbringing on Vulcan. Spock's Vulcan half-brother, Sybok, does not appear nor is mention during these scenes. In Jornada nas Estrelas V: A Última Fronteira (1989), Spock says that he and Sybok grew up together. However, since it's never stated when Sybok joined Sarek's home - only that he did so following his mother's death - or when he was exiled from the family, it's not impossible Sybok moved in after Burnham, and left before she graduated (the two extremes of the flashbacks). Also, since Sybok was never mentioned before Star Trek V, it seems reasonable the family never spoke of him again after his estrangement.
Perceived inconsistencies in the series' use of time travel are not being accepted. Since it is an imaginary concept, the logic can be whatever the writers say it is, even when the mechanisms are apparently self-contradictory.
Despite being a prequel, set 10 years before Jornada nas Estrelas (1966) and more than a century before Jornada nas Estrelas: A Nova Geração (1987), Jornada nas Estrelas: Deep Space Nine (1993), and Jornada nas Estrelas: Voyager (1995), the Discovery is far more advanced than any of the ships from those series (spore drive, holographic displays, automated self-covering spacesuits, etc.). The first Starfleet ship that was seen using holographic communications (as opposed to a standard viewscreen) was the Defiant in Deep Space Nine, set approximately 120 years after Discovery.