I believe that some insurance companies do actually have policies that allow a blood relative other than children or a spouse (parents, maybe siblings) to be included. I remember seeing something about that at a job I used to work at, when they went over the insurance options with me. There was definitely a "your elderly parent can be added" option, at the least. It stuck with me as unique, and a good thing.
I like Kevin's idea of the former spice all signing off on a new spice. That at least implies that they do KNOW their spouse is taking on another spouse, which seems like a good protection for the original ones.
And if the former spouse/spice won't sign...I'm not entirely sure what that would indicate, other than some kind of problem somewhere in the relationship.
I also agree with the idea of taking the marriage benefit away and completely redoing taxing in terms of marriage. Each adult SHOULD be able to self-support, OR, if they're doing a reciprocal support thing (I stay home with the kids, deliberately not having a paying job because childcare is my job), there should be a way to outline that.
When I was part of a polycule, and we were talking through how we'd handle child-reading (two couples definitely wanted kids. Me, Jon, and Lora didn't), we talked about things like how Rachel really did want to be a stay home mom AND had nanny experience with multiple young ones, so would be a good choice. We were going to have a sort of "monthly spending stipend" for her for money, so that she wouldn't have to ask Rob every time she wanted to purchase something. And also, the child-rearing she was doing deserved monetary recognition.
Jessica and George both wanted to continue working, but wanted Rachel to care for their child/children, so they would pay into that monthly spending stipend. Jessica and George also talked about how they might, at some point, want to take time off to stay with the kids, so there was discussion about the possibilities of Rachel going back to work a few years, if someone else switched to the primary care-giver.
Rob (Rachel's partner) works from home, so though Rachel would primarily take care of the kids while he worked, he'd be on hand for emergencies and for if she ever needed extra help.
In addition to all the planning we were doing, it would have been great to have legal protections for everybody too, ones that acknowledged the extended-family situation that we were creating.
Even though that polycule fell through, I do live in hope of another one some day, and the conversations that we all had will definitely serve me well in navigating that again in the future, if I get the chance.