The risk of accidental incest has never been greater after a surge in the number of kids born from sperm and egg donation.
A Cambridge professor believes previous research into families has not taken into account half-siblings in sexual relationships.
Susan Golombok, who has spent years studying the modern family, said a large amount of donor-conceptions happen in the same area.
She told the Hay Festival that many donor-conception children end up going to the same schools or living in the same town.
The Telegraph quoted her as saying: ‘That is a big worry, that children might enter into sexual relationships with their half siblings and not be aware of it.