Her brother claims she agreed to be a donor after he was first diagnosed with the rare cancer, acute promyelocytic leukaemia, in July 2004. He went into remission but then suffered a relapse in February by which time she had changed her mind, he says.
The pair have never been close although their children are similar ages and play together.
Mr Pretty?s wife Jacqueline said: "It is appalling that Helen can stand by and watch her brother die knowing that she could do something to help him. The past few months have been hell."
Mrs Pretty approached her sister-in-law in an attempt to change her mind but lost her temper and was eventually arrested. No charge was brought.
Jacqueline Pretty said: "She opened the front door halfway and I told her that things were desperate and the children thought their daddy was going to die. She said 'Sorry, I am not doing it'. I asked her to give me a reason and she said 'I am putting my family first'.
"I explained that there were no risks involved. I was so upset and I said, 'Don?t you care if your brother dies?' She said 'It?s very sad', and smirked."
The family then received a letter from his sister?s solicitor asking them to keep their distance.
Parent governor Helen, 43, declined to comment yesterday.
She runs a private education business from her ?380,00 home in Wilmslow, Cheshire, which she shares with her partner and her daughter, eight, and son, three.
Mr Pretty, who has two masters degrees, is studying for a PhD in industrial relations while being treated in hospital.