These words are usually found in the jargon of undertakers, lawyers, and the clergy. People’s decease is their death, when they may be said (stiffly) to decease. A deceased relative is one who has died or is dead, and at funerals and will readings, we hear frequent references to the deceased or to the decedent, “the person who has died” (decedent is standard legal jargon). Unless your context calls for distance and ritual impersonality, use die, dead, and death, much simpler words that are not so impersonal, euphemistic, or solemn.
