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A family is a highly interconnected unit. When one member dies, a part of their relatives dies with them – that is, unless their legacy is kept alive. Therefore, first-degree relatives are obligated to mourn. If they mourn their relative in a way that honors his or her memory and inspires them to be better people, the deceased's soul will be preserved in the bonds of eternal life. This in turn gives them life as well. But if the relatives do not mourn properly, they are not helping the soul of the departed ascend in heaven, and hence a part of them dies as well.
The seven relatives who must mourn are mother and father, brother and sister, son and daughter, and spouse. Other relatives, such as grandchildren and nieces and nephews, do not have an obligation to sit shiva. Nevertheless, since they too experience some grief, they should be somber during the shiva (the one-week mourning period) and help provide the mourners with whatever they need.
