Siblings are NOT treated as cohabitants under the Civil Partnership and Certain Rights and Obligations of Cohabitants Act 2010. This Act is the first legal recognition for same sex couples in Ireland, extending ‘spousal’ benefits to gay and lesbian couples in areas of property, social welfare, succession, maintenance, pensions and tax. While it also offers additional rights and protection for other cohabiting couples, including a redress scheme for financially dependent long-term cohabitants on the end of a relationship, it does not cover siblings or other close relatives living together.
The types of relationships that give rise to legal obligation are marriage (spouses); Civil partnership (civil partners) and cohabitation in ‘an intimate and committed relationship’ over specified time periods (cohabitants).  None of these legal relationships apply to parties within prohibited degrees of blood relationship. Only those of the opposite sex can marry, only those of the same sex can enter into a civil partnership. Cohabitants can be either the same sex, or of opposite sex, but can’t be siblings. Both spouses and Civil Partners are treated almost identically under the law.  There are no automatic rights for co-habitants only a right to apply for provision under certain circumstances when the relationship ends, or one of the co-habitants die. Each case will be decided on its merits and on the conduct of the cohabitants.

However, there are tax concessions to all people in inheriting a shared residence, if the recipient does not own any other property and there is a tax relief between siblings that up to a certain threshold is tax free, but any gifts or inheritances between cohabitants are subject to tax on the basis there are strangers in blood.