Neither can I, and if I’ve sounded certain, it’s been unintentional. I’m just saying that, just as I can’t confidently say, “The right will LOVE this proposal!” nobody else can confidently say, “The right will HATE this proposal!”
Let me ask you, then. What rights do you believe you ought to be entitled to regarding your spouse, but an elderly woman ought not be entitled to regarding her live-in sister, specifically?
I reject the idea that extending rights to the sisters opens the door to polygamy, any more than extending rights to same-sex couples opens the door to polygamy. When we’re talking about extending the rights of a mutual contract, there’s no slippery slope between extending them to other consenting adult two-person parties and extending them in some undefined fashion to consenting three-or-more-person parties.