In which I include a loophole allowing me to have characters be nearby when Sarah uses her spell without her knowing about it.
Granted, this is logical and consistent with other abilities in the comic, so this would be the case regardless. It's also applicable to Ellen's copy spell, and is one of several reasons why Ellen copied Nanase with her guardian form instead of Not Tengu (the latter of whom having already easily resisted her transformation beam).
(Other reasons for not copying Not Tengu included the copy spell having short range and a relatively slow transformation speed, but the whole "he could've just resisted it" factor is probably the most important one).
The speed and subtlety of the spell is also important, because spell resistance is much less effective when someone is caught off guard. Even someone with high spell resistance is likey to be affected unless they're on their guard, and since the spell is too fast and subtle for most to notice, it's more likely to be by coincidence than intention if someone resists Sarah's spell.