Ruri is a rich girl. There's no way she isn't going to legally marry Imaizumi for the tax break. That's just how you do things in that income bracket.
Of course, I see no signs this polycule is going to break up any time soon.
Since Ruri is rich they can just move to a country that allows polygamy and they can all marry to him that way.
But if there has to be only one wife I vote Yukina.