Children who don't know the identity of one or more parents run the risk of unintentionally marrying a sibling in later life, Gilmore MP Joanna Gash has claimed.She has cited at least one married couple in the Gilmore electorate, both fathered by an unknown sperm donor, who turned out to be half-siblings.In a speech to Federal Parliament on Monday, Mrs Gash expressed concern over the rising incidence of accidental incest, and called for those affected to contact her office."Some say that the occurrence of this kind of extraordinary situation is on the rise, with the increasing uptake of IVF technologies as well as changing attitudes within our society about children and marriage," Mrs Gash told Parliament."I do believe this is a very serious situation and it can be alleviated if we try to contact the people concerned."Speaking to the Mercury, Mrs Gash said she knew of one case in her electorate in which a half-brother and half-sister married without knowing they were related. It is understood no children have yet been conceived."You wonder how many people there are [in that situation] ... you don't find out unless you have a blood test," Mrs Gash said."I thought it was time to bring it up. Maybe it will start people thinking about where they come from, because some people don't know enough, and the repercussions can be quite severe."Mrs Gash also expressed concern about medical patients who, unaware of their family history, were difficult to screen for genetic-based illnesses.She called for all adopted children to be given their original birth certificates.Mrs Gash plans to raise her concerns with Opposition health spokesman Peter Dutton.Illawarra GP John McAlpine said patients who had been adopted often presented with no family medical history, and had no knowledge of whether they were at increased risks for illnesses with a genetic component such as bowel cancer or heart disease.Dr Chris James, of Sydney IVF Illawarra, said his clinic did not accept anonymous sperm donors."Sydney IVF believes that everyone has the right to know their genetic heritage, and maintaining anonymity for donors prevents this," he said.