A man going through a divorce stabbed his mother-in-law to death, wounded his teen brother-in-law – then slit his own wrists and set their Queens home on fire on Thursday, cops and sources said. The 27-year-old man allegedly knifed his 50-year-old mother-in-law and her 17-year-old son multiple times inside a house on Smart Street near 45th Avenue in Flushing just before 4 p.m., authorities and sources said. The victims were taken to NewYork Presbyterian Hospital, where the gravely wounded