Joshua, now 23, and who suffers from depression, was living as a virtual recluse in the attic at the time.
The court heard he told his sibling that during an argument with their mother downstairs, Mr Tulley had threatened to go and get an axe from the garden shed.
Jerome said: "He (Joshua) told me we had to kill him."
Mark Wyeth QC, representing the young defendant, asked: "Did you agree?".
"Yes," he replied, adding: "I saw that as the only option, because he was a threat to everyone in the family, including me, my brothers, my mum and even his own children."
He said he and Joshua went downstairs and got two knives from the kitchen drawer before "attacking" Mr Tulley as he lay on the sofa he slept on in the living room.
"I wasn't thinking properly and it was all a blur," said Jerome, who could not recall how long the attack took.
The jury heard that afterwards the pair had left the house and walked to a nearby forest, still covered in Mr Tulley's blood and with the knives in a Sainsbury's bag with baby wipes the pair claimed were for covering their injured hands.
"There was a discussion of suicide, I don't remember who started it," Jerome told the jury. "I considered it. We were going to commit suicide with them (the knives)."
At 9.15am the following day Marie Ellis called her mother-in-law, Patricia Ellis, and told her Neil Tulley was dead.
Mrs Ellis told her to ring 999 but it was not until half an hour later that she did, lying to ambulance controllers that she was calling about a heart attack victim.
When paramedics arrived at the property, in Guildford, Marie Ellis took some time to open the door and then led them to the body on the sofa.
There were still three young children in the house at the time.
The brothers evaded police for several days before turning up dishevelled at a church a few miles away, where the older brother told a bellringer: 'We're murderers' and asked him to contact police.
Prosecutors have already outlined how Mr Tulley had been acquitted the previous year of attempting to murder Joshua by swinging an axe at him.
Jerome and another brother had been kicked out of their natural father Gary Ellis' home on January 29, 2012, after he discovered they had been sneaking off to visit their mother and Joshua in secret.
Marie and Joshua were said to be happy they had returned, but Mr Tulley had argued that they could not stay in the small home already occupied by him, his partner, Joshua and Mr Tulley's three daughters with Marie.
As the argument became heated, Mr Tulley was alleged to have walked out to the shed and picked up an axe, bringing it inside and threatening Joshua.
"I saw Mr Tulley try to kill my brother with an axe," said Jerome.
"He lunged at him," he added, demonstrating a chopping motion.
"It didn't connect, Josh knocked it out of they way and tackled Mr Tulley to the floor."
Once Mr Tulley was arrested, tried and eventually acquitted, he returned to the family home.
The court heard he drank daily and was abusive towards Joshua, Jerome and Marie regularly in the lead up to them attacking him.
Justin Rouse, defending Joshua, said Mr Tulley had previously had a restraining order taken out against him by Marie and had also received a caution for strangling her.
Philip Katz QC, prosecuting, questioned whether Mr Tulley had in fact threatened to use the axe, which Jerome had seen after breaking into the garden shed a week earlier.
He asked why the police were not called during the argument, to which Jerome said: 'We had tried the police before, he was acquitted and came back to the house and continued to be abusive.
"I wanted to protect my family and if that meant doing that (killing Tulley) then that was the only option."
As father Gary Ellis sobbed in the public gallery, Mr Katz QC questioned Jerome's recollections and said he was trying to protect Joshua.
"Things were going from bad to worse in that house after his acquittal and I suggest that you were angry," he said.
"An element of revenge was involved in what subsequently happened. On the night in question the decision was made that Neil Tulley was to die and you agreed to take part in that."
Jerome denied any planning or element of revenge in the attack.
The trial is expected to finish on Tuesday.
- Daily Mail
