Relatives of a Roman Catholic priest kidnaped Jan. 8 in Lebanon said they have received a handwritten letter from him, raising hopes that he is alive. Father Lawrence Martin Jenco's brother, John, of Joliet, Ill., said the letter was dated Feb. 19 and was received Friday. He declined to reveal its contents. The priest, 50, head of Catholic Relief Services in Beirut, was seized by gunmen on his way to see a doctor about a heart condition. He is one of four Americans believed held by guerrillas in Lebanon.