Zach wasn't planning to involve her in the first place, because if things went well, she wouldn't even be aware that her brother's life was threatened.
However, contrary to his expectations, Edna did not utilize direct physical attacks. Thus, he had to monitor Luke as closely as possible.
Whatever happens, it would probably transpire inside him because based on the books he read before, sirens who had the ability to impact or modify their target's environment through their song, so they could let them absorb the curse through their bodies, can also set specific situations or words for the curse to take effect…
And the impact, it was only described that it could be mental like hallucinations or dreams, or physical, wherein it would trigger imbalance within the body strengthening weakened bodily organisms that could lead to disease and in extreme cases death.
