I was "invited" to step into a previously existing poly-couple relationship, where he was heterosexual, she was bi-sexual and I was heterosexual. So, basically it was a "vee", when I stepped in.......although it was "open" such that any and all parties would be free to establish other relationships. (I wasn't particularly interested in establishing other relationships, so I probably could have fit into a relationship similar to what you're discussing. I wouldn't have been a unicorn, however, but your "sister-wife".)
I found it extremely challenging to find my way into "their" relationship. They had a few years together and had worked through many of their relationship issues. Although they didn't intend it to happen, I ended up being the one to do most of the fitting in and adjusting because they had a life worked out together that worked well for them. Also, she had an 8 year old son from a prior marriage that they were raising, so we all had to adapt/adjust/accommodate to his needs, too. For example, attending his swim meets, need for childcare, need to be transported to and from.......,etc.
They would make decisions that could ultimately affect me (for example, committing to certain events/activities that would mean less time for me to spend with them/him.) and never think to consult me because they just in all honesty weren't used to having to think about the impact on an additional person. When I'd bring it to their attention, I'd often be seen as being "jealous" of her or her son or ??????
Ultimately when he got so stressed with his job and other time commitments, I got left out. He decided he wanted our relationship to be "fun" and "casual" (which equated with "secondary" in my book, and I had NEVER wanted to be a secondary and had said so from the start.) I decided not to settle and broke the relationship off. It was a profound loss for me. Not just the loss of him, but of my friendship with her and with their friends and family.
I'll give you a lot of credit for being upfront and clear about what you want with someone. But, as others have mentioned, there are many pitfalls for someone stepping into an well established relationship in spite of your best intentions. I, for one, would also feel very "used" or insulted to think that one of my values in a relationship was to help provide a buffer to the feelings of loss anticipated if one of the original partners "who were so in love, so committed to each other that they couldn't imagine going on without the other one"...died. Many two person monogamous couples love each other to this degree.....and they just have to deal with it. Afterall, no one can ever replace another, or totally understand what someone else is going through regardless of how close and intimate you are.