A6-year-old child has been wetting the bed at night since the birth of a baby brother. what defense mechanism is the child using to cope with the stress of accepting a new family member

The answer would be: regression

Regression is a defense mechanism that causes the person fall into earlier psychological stages when an unpleasant stimulus received. The person chooses to turn back rather than facing the problem, which was seems safer and easier.
In this case, the 6-years old child starts to wet his bed, a sign that should be found in the baby(earlier stage).

It was said to have happened by earthquakes, the shifts in the tectonic plates moved everything