DeathKorp_Rider wrote:Are the
SoS considered to be part of this faction? And would this allow them to be included in an Astra Militarum faction?
Assuming you're not referring to some new Psychic Awakening event stuff I haven't seen yet since I don't keep up with Warhammer Community directly, no, they aren't. Their Faction Keywords are IMPERIUM, ASTRA TELEPATHICA and SISTERS OF SILENCE. You cannot include them in an ASTRA COPYWRITUM detachment (in Matched Play). You can, however, include them as part of an IMPERIUM army, so long as you isolate them to their own detachment.
Your options are to either put them in a special no
HQ Vanguard (as described by their "Null Maidens" ability errata) or to put them in a normal Vanguard using a Primaris Psyker from Codex: Astra Copywritum as your mandatory
HQ selection, as the Primaris Psyker has the ASTRA TELEPATHICA Faction Keyword. You cannot use a Primaris Psyker to "link" a detachment together (in Matched Play) because the rule requires all units to share a Faction Keyword with ALL other units within the detachment, not just at least one other unit.