HAWTHORNE — A 20-year-old white man was arrested Thursday and accused of burning a cross in a black family’s front yard. The suspect’s sister had been involved in a school fight with a 15-year-old boy from the black family just hours before the cross appeared last week, Capt. Keith Corlett said at a news conference. Corlett announced later that the boy was also arrested, on a juvenile assault charge. The cross-burning suspect, Christopher Hudak of Cortlandt Manor, was arraigned on a charge of aggravated harassment. Westchester County District Attorney Janet DiFiore said the case would be prosecuted as a hate crime. No plea was entered. A call to Hudak’s lawyer, John Sullivan, was not immediately returned. Corlett said cross burnings have historically symbolized racism in “a graphic and grotesque manner.”