You would not be a sister wife.
He would be... kinda... but your partner is not his brother. Or do you want to be with your sister's spouse? Or be the other wife to a couple? Like an "N" or a "W" config?
I don't think you mean that. I think you mean YOU are the hinge. And your DH is ok with that. But he doesn't want any other loves, so just you is it for him.
To him then they'd be metas or metamours. "My lover's other lover."
Better still -- call it a "closed poly-fi V" structure. I think that is maybe what you are after and I don't know your plan to dating but getting some vocab down better could help you succeed in finding what you want. Esp if you are writing personals and stuff.
He is married to you, you could not actually be LEGALLY married in some places to your Other Significant Other (OSO) but could have a poly commitment ceremony for spiritual/emotional reasons if/when that gets to that place. I think that's what he is saying by being "open." Is that what he is saying?
But basically I get the vibe you are thinking like it would be a closed thing to just you three in the end of your dating time to find the right fit, right?
With you as the hinge with the two spouses, and the arms of the "V" being friends. He says he's not open to more spouses, so he's not looking for a triad situation. Then it would be like 3 "V" laying on top of each other because each person IS involved with the other two romantically, sexually etc.
Anyway, that's what I'm hearing. I'm not sure.