A family feud came to a head when a businessman smashed his sister's window at an historic country house where they live in different wings.
Magistrates heard Philip Baden Watkins, 47, and his sister Lynn Zijlmans share the restored Flaxley Abbey near Blaisdon in the Forest of Dean.
But the relationship soured and they fell out over shared business dealings.
Mr Baden Watkins snapped after he returned home one night with his family and saw his sister approaching.
He smashed a window and damaged a fence at the 12th century property which was bought in 1960 by industrialist Fred Watkins – the pair's grandfather, who later left it to them – and it was restored by the acclaimed stage and property designer Oliver Messel.
"He is in business with his sister and they live in separate wings. It is a large property," prosecutor Karen Grant told Stroud Magistrates' Court on Monday.
"It appears there have been difficulties and they no longer speak. They were in business but it is being sold.
"The defendant returned home with his wife and family on the night in question, and he smashed up his own post box, then witnessed by his sister he damaged a fence. He walked across the garden to her wing. She feared for her safety and he smashed a window."
Jennifer Stetson, defending, said they owned a business together.
"It came to having to sell the business and to say it is acrimonious is an understatement," she said.
"He tells me difficulties became more acute with the approach of his sister and perhaps her wish to sabotage the sale."
She said on the night of the offence he "snapped" and did something he had never done before.
"He deeply regrets his actions," she said. "It is a very sad situation."
At a previous hearing Watkins admitted criminal damage at a the house which dates back to a Cistercian abbey founded between 1148 and 1154 by Roger, Earl of Hereford.
District Judge Joti Bopa-Rai gave him credit for pleading guilty at the first opportunity and said: "It's a shame you have to come before a court at your age. I will give you a 24 month conditional discharge."