For whom must we say observe the laws of mourning, including reciting Kaddish?
The answer is that we are obligated to observe these laws for seven relatives, namely, father, mother, sister, brother, son, daughter and spouse. This list of relatives is based on the beginning of Parashat Emor in the Book of Vayikra (Leviticus) in the Torah (chapter 21, verses 1-3). There, the Torah tells us that Kohanim, who are to avoid contact with the dead, are nevertheless permitted to be involved with funeral arrangements for their immediate family, and it specifically mentions these relatives (the spouse is not mentioned in these verses, but was added to the list at a later time).