OP/Blackheifer criticisms are fair.
During the dialogue when Marcus reveals his intent, you do have time to reposition companions, go turn based, repick spells, and cast short term buffs. In this way you can have multiple Sanctuaries prepared and up on Isobel already.
Without a cleric/devotion paladin, you might be able to use Invisibility?
But yeah, she's not an ally (fair) which has all kinds of tactical implications. And her combat AI is... inexperienced.
I've seen her go down to two flying ghouls; one paralyzes and the second auto-crits. With her AI as is, it's possible she breaks Sanctuary/Invisibility and then gets ganked. I suppose a blocking wall of crates is the best answer for this.
