Their leader ability states:
"... If all allies are Nightsister at the start of battle, at the start of each encounter and end of each turn, if there are no enemies inflicted with Doomed, Great Mothers removes Ability Block from themselves, gains the granted ability Fate Weaving, and takes a bonus turn. ..."
However the ability doesn't actually include any check that the entire team is Nightsisters before granting a bonus turn. It effectively only checks that an enemy doesn't have doomed and then places the leaderability_greatmothers_doomed_marker on Great Mothers, triggering the bonus turn at the end of every other character's turn. The two granted abilities cannot be used because they are correctly gated by hiddenability_greatmothers_all_NS_marker.
You can test this yourself by taking Great Mothers as leader with any team composition that has at least on non-Nightsister. They will forever take a bonus turn after each other character's turn and aren't forced to the granted abilities. See also this video clip.
