A man in his 40s was found fatally shot near the Colnbrook bypass, Slough, on Monday.
A relative said Reuben Gregory had lived with his sister Kathleen in the area for more than 50 years. It is believed they lived in a caravan.
She described them as "very, very quiet inoffensive people".
"They are just very peaceable people, living a very simple life. That's the way they have always lived, that's the only life they know."
The family member, who did not want to be named, said the siblings had been targeted 30 years ago in a burglary during which Ms Gregory was tied up.
Emergency services teams, including armed officers, found the man at 03:30 BST.
Mr Gregory, in his 70s, and Ms Gregory, in her 50s, remain in custody.
Det Ch Supt Chris Ward, of Thames Valley Police, said investigators were treating the shooting as an "isolated incident".
He added: "I must stress that this incident is not being treated as related to terrorism and there is no threat to the wider public."
On Monday, officers arrested a 48-year-old man on suspicion of burglary in connection with the incident.
The man, who is said to be local to the area, is in police custody.
An elderly man and his sister arrested on suspicion of murder live together in "secluded" Berkshire woodland, a relative has claimed.