As regards hospitals, it was a big issue for many gay men during the AIDS years, when partners of many years would find they had no rights to even bury their partner, once unsympathetic family had arrived on the scene.
What about responsibilities as well as rights? I have a different viewpoint, since for many years I thought I would never see gay marriage. So when the option arrived, we had to say, what are the rights and responsibilities? There is more involved than just tax breaks, you take on responsibilities as well! Plus there is the social capital and frankly the recognition you get from being married! People at least know that it means some level of commitment. I guess that is the difference between marriage and siblings co-inheriting -- you choose your partner, you can't choose your family!
Plus the 'two sisters' scenario is always rolled-out by the right-wing press, because frankly signals to me that they don't see lesbians as a valid relationship or family, hence this direct comparison as being either equally valid or more valid. But if they don't take on any responsibilities for each other (Would your rights to benefit be altered if you declared you shared a house with your sister? Would you expect her to support you financially if you lost your job?), they why should they get the benefits that (some) married people do?