If you think gay relationships are as real as straight ones then supporting civil partnerships for sisters means you must support marriage for a brother and sister.Now note that the vermin is suggesting that the commentor they are castigating hasn't thought it through. Which is, at the least, egregiously ironic.
If you support a legal partnership for aged sisters, you probably mean it without the lesbian incest. (Ed notes: there is an assumption there. But we'll ignore it for decency's sake.) In which case, you could be assumed to support a similar legal partnership for a brother and sister, without the heterosexual incest.
And, frankly, I do - and for non-bonking friends, regardless of family relationships. Particularly on the "next of kin" grounds - who would you rather make critical medical and legal decisions for you if you were incapacitated? Somebody you have lived with for years; a nephew or niece you haven't seen, except for funerals, since their 12th birthday; or the state?