Question:

Why do we mourn our parents for a full year and just one month for all other relatives?

Response:

Jewish law mandates the one-month mourning period (called sheloshim), after which a person is not obligated to continue any official mourning practices (although the grief does often take much longer to abate). The extra mourning time mandated for parents has nothing to do with the level of loss or grief; it is simply an extension of the special mitzvah to honor the mother and father who gave us life.

Chaya Sarah Silberberg