It seems like it is an exceedingly messy situation all around.
I can't advise you what to do but I can tell you what I would do in that situation and that is stay the heck away from anything like that.

Are you stable? Is she stable? Are all of your selves in agreement with this relationship or are some of you against it and some of you ambivalent? Are all of her selves in agreement with this relationship or are some of her selves against it and some of them ambivalent?

I think relationships work out better when there are clear boundaries. Im the situation you mention - 1 alter in you and 1 alter in her are in love but the other alters are uncomfortable or against it - how on earth are you going to figure out the boundaries of what is okay at what time and with whom?? How on earth do you protect yourselves???