How should this be put to family members who insist on the "But the two of you are faaaaamily! Thats the only sister you'll ever have!" mentality? What is the polite way to put it that doesn't blame LS or me, but makes it clear that we want nothing to do with each other based on mutual decision? That some people, despite being family, just dont' get along?
How does the topic come up? I think the tactic is to deflect it in the first place.
"Have you talked to sis lately?" -> "Can you believe how big the kids are?" instead of "We don't talk much."
Or "How often do you see sis?" ->With the kids you can imagine how busy things are, but we are happy with the amount of time we get to spend together." instead of "We have nothing in common so as little as possible."
This is what I wonder as well. If you are sitting there discussing the nature of your relationship and how you don't get along, then it is a little much to then tell someone it isn't their business when they give you their opinion. On the other hand, if they just bring up your sister with the assumption that you will want to talk about her or know about things in her life, just deflect with generalities and change the subject.
Since you don't have aunts, uncles or cousins, I'm wondering how it comes up. I suppose someone would ask in passing, but just don't give them any detail.
And remember that you do NOT have to answer the question they are asking--not the literal "words out of their mouth" question, and not the unspoken question behind it.
When people ask about your sister, say things like, "She's fine. I've been busy lately, so we haven't spoken recently."
Also, feel free to say, "why do you ask?" Then when they say, "Oh, I'm just wondering, and hoping she's well," you can say, "She is fine."
(which is honest, because if she were really sick, or had disaster strike, you'd have heard through mom and dad, right?)