As much as I'm sure you'd like to get in her face about it, that would be counter-productive to do so. It is up to him to inform his other partner that you are now also his partner.
I have to wonder... since this issue has arisen just what he HAS told the other SO. Has he told you that he's poly but not her? Do they have an understanding that there will always be a triad or quad or what-have-ya between them and their partners?
It seems to me that there is probably a lack of communication going on.
Pinky, 41, f, bi. Married to Bear, 42, m, straight.
Not dating anyone right now.
I became insane with long intervals of horrible sanity.--EAP