Jamie Batten from Hawick was married to hanged Harris's sister Jan and set-up home in the town. They later divorced and she re-married.
Police found Harris dead in the garage of his run-down home in Livingston seven days after 11-year-old Rory Blackhall was found murdered in woods close to his home and school.
And it later became known that the 37-year-old had been charged with abusing three girls between 1993 and 2000.
Mr Batten says that if Harris was a killer and a paedophile there should be no mourning.
The former Roxburgh District Councillor told TheSouthern: "If these things are proved, then he was a man of the devil and should not be buried with decent Christian folk. His body should be cremated with no ceremony and his ashes just thrown away.
"I have spoken with Jan and she agrees that his family should not attend any funeral and be seen as mourning a man who killed a child and then took the cowardly way out and killed himself. He should be given no rest and should roast in hell."
Harris's sister Jan and Mr Batten were wed in May 1990 in the first marriage ceremony to be held at Jedburgh Abbey in over a century. They became engaged two weeks after meeting and married after two months.
Harris did not attend the wedding but shortly afterwards spent several days with his sister and new brother-in-law at their Hawick home.
Mr Batten told us: "He helped put up some shelves in a bedroom was eloquent, perfectly well behaved and well put on. There is talk now that he had been taking drugs and been interfering with children. But there was no indication of that at the time. If there had been he would not have been allowed through the door."
Mr Batten appealed for the Blackhall to be left alone to mourn and for remarried Jan and the family not to be hounded over what had happened.
"It is not fair for the rest of the family to be left to pick up the pieces as a result of what he has been accused of doing," he told us. "It is not their fault.
"Jan is a lovely person and should not carry the can for the supposed actions of her brother."
In the early days of the Rory Blackhall enquiry criticism was levelled at the local education authority for failing to notice that he had not arrived at school after being dropped off by his mum.
And TheSouthern can real that four days after Rory's body was found a 12-year-old pupil from Earlston High school was spotted wandering by the A68. He told a burger van operator he was heading home to Gattonside. Police confirmed they were aware of the incident.
Leona Bendall of Scottish Borders Council admitted to TheSouthern: "A pupil, who was seen coming to school by fellow high school pupils, did not appear at registration and an alert was raised. School staff took steps to find the pupil and did. Then after a conversation with the police, who had been separately notified by a concerned motorist, and the pupil's parent, the child returned to school."
And she revealed that Holyrood-funded trials on a truancy call system would be starting soon in one primary and three secondary schools.
She told us: "If a child is not in school at first registration and we have not had a call from the parent or advance notification of an absence, an automated response message would be sent to the parent.
"The automated response can take the form of recorded voice message, text or email – depending upon the parents' preference."
At the moment a variety of systems are operating in schools across the Borders.

Meanwhile, it has become known that the sheriff who granted Simon Harris bail on child-sex charges was former Border solicitor Martin Eddington.