Hugh Bonneville wanted to "kill" his older sister and even began digging a grave for her.
The 'Downton Abbey' star admitted he was tired of his sibling Claire picking on him around the house when they were children, with her "unnecessary" behaviour including kicking him under the table and sticking her tongue out at him, so he told his mother he wanted to do away with her, but she didn't take his threats very seriously.
Speaking on 'The One Show', Hugh, 59, said: "Anyone who's had an older sister realises at some point that they need to die or be killed. I think I was about seven or eight. Claire had really annoyed me.
"So at one point I did say to my mum, 'I'm going to kill my sister' and she said 'Alright dear, dinner's at six.'
"I said 'No, no, you've got to understand. I'm going to go and dig a grave now.' She said 'Alright, it's fish fingers. Is that alright?'
"So I marched out and unfortunately the soil in Blackheath, south east London, was a little hard, but i managed to get about four inches down."
The 59-year-old actor is glad he didn't go through with his plan and Claire is still "alive and kicking".
He added: "Anyway, I'm glad to say that I didn't actually follow through and I love my sister very much."
Meanwhile, the 'Paddington' actor previously insisted Harvey Weinstein's films should not be ignored in the wake of his imprisonment because the producer created some "memorable art".
He said: "I think there is a pendulum swinging at the moment to do with cancel culture - that all the films of Harvey Weinstein should be burned or something.
"But actually, the art that he produced was memorable in our contemporary culture.
"So I think there is a discussion to be had certainly, and one that must hold these things in balance without condoning in any way the [offensive] views."