He said the athlete has two children whose father is a different person living in Uganda.
The father said the two had a case which was being investigated by the DCI.
Mzee Cheptegei said the two had separated for long but on Sunday the man sneaked into the home where he staged the attack.
Chelagat broke down outside the ICU as she explained the condition of her sister.
Cheptegei participated in 10,000m race at the recent Paris Olympics.
She was critically injured in the attack at her house at Kinyoro in Endebes Trans Nzoia county on Monday morning.
Police said she was attacked by her alleged boyfriend who doused her with petrol and set her on fire
“The man was also caught in the same fire and sustained serious burns,” said Trans Nzoia police commander Jeremiah Ole Kosiom.
He said investigations established that the two had been frequently quarrelling over a plot and a house.
On Sunday the man is said to have sneaked into the home with a five-litre jerrican full of petrol.
Cheptegei had gone to church with their children and when she returned the man poured the petrol on her and lit fire.
Neighbours had a commotion and rescued the two who were rushed to hospital in critical condition.
