I don't understand why he hasn't taken you out with him to meet her; afterall you are the one he is going to marry. Plus he's not a single man any more if he's engaged he shouldn't be going out with other women at all.

I'd make a point of it to him that he shouldn't be putting his arm around her and he shouldn't be creating intimacy if he is in love with you; I think his reaction is because he knows what he is doing is wrong but doesn't like being called on it.

You deserve not only fidelity but total honesty and openess; this woman can be a friend to both of you or neither of you and there is nothing wrong with saying that,

Supporting you totally,

Rhian