What does the writer warn us about?

The answer is Peter

Explanation:

Peter: He was a boy at the age of sixteen that was with his family hiding in the Secret Annex with the Franks. He has a quiet appearance but finally warms up to Anne during their two years of hiding.

Anne's first feeling about f Peter was not in any way kind, she described him as an awkward, shy boy going on sixteen, whose company offer much gain to her.

The situation peter must have found himself and to have   been thrust into a small space with a family they don't know much of, which must have indeed made things uneasy for a while.