flip the question, aren't you trying to use magick to sway your fiance's opinion? you should talk about it, and while she's fine on paper, ask why she doesn't want to go through with sister wives? everyone has their own unique life, but sister wives mean you're marrying two people, not that the three of you are married. sister wives love their husband like a spouse, but they love their spouses wives like a sister. it carries a whole array of conditions [specifically emotional, but legally too] that can make it difficult. your fiance might feel deep down she's not enough for you, or perhaps you're the dominant partner, and not an equal. it would take a lot of trust for it to work out. a friend of mine [now ex wife] suggested they have an open relationship, they agreed on some ground rules and went for it. she dated several other guys over the course of 4 years, but the only time he wanted to date someone the wife hit the breaks on the plan. my point is dig deeper, spend time together as a married couple before throwing a third person into the mix. perhaps instead of a sister wife, your fiance is fine with an open sex life. only theories of course.
regarding spells, you can try making a honey jar spell to open her to the idea, or perhaps a relationship spell to find someone you both like [try casting it together. not specifying a person, but personality]