If domestic partners or gay marriage is allowed, does it not apply to anyone including brothers or sisters?
If all government actions must be without bias of sex then all claimed partners must be without consideration of any sex.
This then allows for tax exemptions for any one a person decides to name as a partner. All legal partnerships must have tax treatment of marriage. This includes business partners.
This whole thing becomes just a general tax break. Estates can then be passed from person to person without estate taxes. Anyone can then be named as a dependent.