We are obliged to mourn for a father, mother, son, daughter, brother, sister (including half-brother and half-sister) husband or wife, but not for an infant less than thirty days old. Thus, in-laws and grandchildren are not designated as mourners. Males from the age of thirteen years and one day and females from the age of twelve years and one day should observe the laws of mourning.
We are permitted to observe the mourning rites for anyone, not only for the relatives listed above. The Hebrew term for mourner is Avel, the plural, Avelim.