Daniel Schofield, 29, denied the charges after Roy Hanson, 50, died in October last year.
During the week-long trial at Truro Crown Court, Mr Schofield maintained his death was an accident.
Police were called to a property in Trevallack, outside St Keverne, near Helston, Cornwall, in October.
Mr Hanson was treated at the property when he was found by paramedics, who then called police.
Mr Hanson, a plumber from St Keverne, was married to Mr Schofield's sister Julia Hanson.
The court heard he was violent towards her and she left him, staying with Mr Schofield.
But Mr Hanson arrived at Mr Schofield's farm on the Lizard on 25 October last year, the court heard.
The trial heard he was shot with the crossbow while attacking Mrs Hanson.
Mr Schofield told the court he was hoping Mr Hanson would feel threatened by the weapon and leave.
Questioned by his defence counsel, Richard Smith QC, Mr Schofield said he had no thoughts of firing the crossbow when he loaded it.
It was discharged and the bolt hit Mr Hanson in the chest, passing straight through his heart and a major blood vessel before exiting through his back.
They were also shown the blood-soaked bolt and the crossbow that fired it.
A statement from Mr Hanson's family said: "Roy was a loving, devoted and loyal family man and would have done anything for anyone.
"How he has been portrayed is not his true character. Anyone who knew Roy would know who the real man was."
It took the jury nearly five hours to find Mr Schofield not guilty of the charges.
A man accused of killing his brother-in-law with a crossbow has been acquitted of murder and manslaughter by a jury.