Any real reason why Lisa changed when she did?

I would hazard a guess that maturity quickens the process. We did not see any nonadults of the Family. While the boy was going through the slower process, she fought mentally. Once he recovered, she relaxed and stop fighting.









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Is it the Devil in the whiskey, or is it the Devil in him? -- ???

The is foreshadowing as to what is going to happen to Lisa. She needs dark sunglasses when she goes out shopping and when she wakes up after having spent the night with Neville she has to close the curtains to block out the morning sunlight because it's painful. At least that's how I interpreted it.