Two sisters in their eighties will argue in front of European judges this week that the law treats them less favourably than a lesbian couple.
The siblings have been living together in Wiltshire since they were born, but Joyce, 88, and Sybil Burden, 80, fear that one of them will be left with a large inheritance tax bill when the other passes away.
In 2005 the implementation of the Civil Partnership Act brought inheritance laws for gay couples in line with those of married couples, meaning a surviving spouse will inherit their partner’s estate without paying tax.
The law does not apply to family members living together, which Joan and Sybil claim is a contravention of their human rights.
In December the European Court of Human Rights rejected the pleas of the elderly sisters asking for the same inheritance tax rights as married and gay couples by four votes to three.
On Wednesday their appeal to Strasbourg’s grand chamber of 17 judges of the ECHR will be heard.
PinkLegal.co.uk barrister Alistair Kelman said: “In fairness they have a good case, this highlights the unfairness of inheritance tax.
“The best thing to do would be to abolish the spouse and civil partnership exemption but raise inheritance tax so that it would hit the very rich.”
The spinster sisters have written to the Chancellor before every Budget since 1976 asking for exemption for family members from inheritance tax.
Last year they also wrote to the European Court of Human Rights and were shocked when the court agreed to hear their case.
When one of them dies, the 40% inheritance tax on their family farm would mean the remaining sibling would have to sell the property to pay the bill.
Their four bedroom house and 30-acre farm, near Marlborough in Wiltshire, are estimated to be worth £875,000. Only properties worth more than £285,000 are subject to inheritance tax.