Theresa May last week bowed to mounting pressure and announced prospective legislation to extend to opposite sex couples the right to enter into a civil partnership, previously an option only for same sex couples.

With marriage now permitted between partners of the same sex, the number of civil partnerships has dropped dramatically. Marriage was what same sex couples wanted; civil partnership was seen, perhaps, as almost but still not quite the same.

As my partner Stephen Parkinson observed in his recent blog, a proportion of new civil partnerships represent the choice of same sex couples only where marriage might carry a career risk; among Church of England clergy, for example.

If I’m honest, I’m really not persuaded by the arguments put forward for opposite sex couples for wanting a civil partnership rather than a marriage. And I can safely own up to that because, while both marriage and civil partnership are 'legal' arrangements, their emotional, spiritual, and political meaning is a personal thing. There’s no right or wrong.

I also wonder how many people really desire to put a relationship that is purely platonic or simply practical (flatmates) onto a legal footing? On the one hand, that legal relationship gives inheritance rights (and an Inheritance Tax exemption) but on the other throws the parties into a situation akin to divorce if they split up.

While I still wonder why a committed couple would choose civil partnership over marriage, I nonetheless welcome this change in the law – because it recognises that meaningful relationships can take many forms.

I’m hoping that the current attention given to the legal status of relationships will lead to a re-examination of the legal and practical issues facing co-habiting couples.

The most recent Cohabitation Bill has ground to a halt . Yet the current situation where the legal status of cohabitating couples’ relationships has to be cobbled together by judges, using a rag bag of legal tools not designed for the job, urgently needs addressing. As of now, judges have to push the boundaries of the principles of Property Law, Equity and “Constructive Trusts” in their attempt to achieve some degree of fairness when cohabiting couples separate.

One aspect of the civil partnership debate which is only now receiving increased media attention is whether same sex siblings should be permitted to enter into a civil partnership.

The debate really took off ten years ago when the elderly Burden sisters, who had lived together for over 30 years and owned their home jointly, challenged Britain’s Inheritance Tax Regime in The European Court of Human Rights. They argued that denying them the relief from inheritance tax given to civil partners and married couples was discriminatory. They lost. Their relationship had no legal status and a Government was just in giving tax breaks to legally recognised, respected and socially encouraged relationships.

Many commentators at the time (me included) felt that the sisters chose the wrong basis in making their Court application; they should have argued that denying sisters the right to enter into a civil partnership was discriminatory.

Marriage between 'prohibited degrees of relationship' has long been a nullity; there are clear moral, biological and genetic reasons why close relatives shouldn’t marry, especially when coupled with the legal requirement for a marriage to be 'consummated'.

A valid civil partnership requires no consummation; likewise 'adultery' (which is a male/female affair) isn’t a ground for dissolution.

It’s thus hard to fully understand why civil partnership between sisters, brothers or other close relations of the same sex is prohibited. The BBC website has picked up the issue with an interview with Catherine Utley and her sister Ginda.

And they quote Conservative MP Sir Edwin Leigh: "Why should siblings who've lived together for years have to pay estate duty when one dies?"

And that tax issue, I’m guessing, is the real driver behind the Civil Partnership Act (Sibling Couples) Bill which received its second reading in the House of Lords earlier this summer.

The Bill would extend the right to enter into a civil partnership to siblings - but only if they are both over 30 and have lived together for 12 years. This really does smack of a change to tax legislation (extending eligibility for the Inheritance Tax Spouse Exemption) rather than reflecting a need to recognise a relationship on the back of a sea-change in social attitudes.

Nevertheless, a change in the law on civil partnerships giving legal status based on a defined period of co-habitation may prove a step towards a fully formed law, so desperately needed, that protects the rights of all long term cohabitees.