Some thoughts: only the first two books are canon for this game. (That doesn't mean a few bits of stuff after won't show up, it just means characters can't assume they know about it).

I'm pretty sure in one of the other threads (Powers, maybe?) I mentioned that magic doesn't pass through Trump contacts. You get only the mental contact and purely physical contact.

QoA's reasoning is one valid way. She has enough of the relevant skills/stats to have the info to reason about the situation. And Michael's assumptions are not entirely wrong here; a faster execution against a low-psyche/non-spellcaster sibling could work.

More devious reasoning: Sakura has a psyche advantage, may have a spellcasting advantage, and knows Michael well. The delay, plus the slight hint of him going into / being in mental "spellcasting stance" is enough for her to bluff, and then his reaction is enough to confirm the bluff