Following the introduction of civil partnerships in the UK the same question arose about lifelong 'partnerships' between siblings.

Elderly sisters take inheritance tax fight to Strasbourg Sept 2006

'Joyce and Sybil Burden, aged 88 and 80, want the same rights as married or same sex couples. As the law stands, when one of them dies the other will have to sell their home in Marlborough, Wiltshire, to pay the inheritance tax.

The 2004 Civil Partnership Act extended to gays and lesbians the same exemption from inheritance tax already enjoyed by married couples - but it does not cover family members living together like the Burden sisters..........

Sadly, though, civil partnership is not an option for sisters, just as marriage is not an option for a brother and sister.)'