Using inference, who may the reader assume died last year during the lottery?
The only character that the text indicates as having a missing family member is the Watson boy.
The rules indicate that the man of the house draws the ticket for the family. If the man cannot make it, such as in the case of Mrs. Dunbar, whose husband has a broken leg, a grown boy of the household then draws. If there is no grown boy--again, as in Mrs. Dunbar's case, as her son Horace isn't more than sixteen yet--the woman draws.
Mr. Summers asks immediately after that exchange, "Watson boy drawing this year?" suggesting that he knows there is no Mr. Watson; he just isn't sure of the boy's age. A tall boy steps forward and announces that he's drawing for his mother and himself. He seems nervous, too--more so than the other villagers--as he goes forward to select a ticket. The villagers say he's a "good fellow" and they're "Glad to see [his] mother's got a man to do it."