Mo Huanjing, a former nanny of the family of Lin Shengbin, was found guilty of arson.
Lin's wife and their three young children were killed in the incident last year, the Hangzhou Intermediate People's Court said in its verdict.
Mo started the fire using a lighter in the early morning after she searched the keyword "arson" online, according to the court.
Mo was also convicted of theft, state-run Xinhua news agency reported.
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