Thanks for posting this BP - and thanks to your sis and partner for sharing what works so well for them.
This created a bit of a lightbulb moment for me. Reading about how your sis/partner protecting their core relationship and the steps they take to make sure it isn't threatened triggered some feelings of being trapped in me.
I've been thinking about why that might be and I think that it reminded me of the difficulties I had while in a mono relationship with being emotionally close to other people. My ex just couldn't stand it - he was so upset by my close relationships.
Gradually I spent less (or no) time with those people until eventually the only person I was emotionally close to was my ex. It felt horrible to me, just horrible. I reckon I was depressed for years and certainly after we had split up and I'd gotten over the distress of that, I could not have been less interested in finding a new relationship.
I've thought for years that I have a problem with monogamy - but maybe I don't. Maybe I have a problem with being restricted in who / what I am able to be emotionally close to??
Anyway - thanks for posting that. I found it really helpful.