I'm going to guess that the reason she is so concerned with you and him being in love is because she (and he, and they together) have NOT fully worked through the past loss of trust issue. He lied and cheated on her with other people, and in some place she "accepted" that's what he needed and has opened up the possibility of him having that in their life to keep her family intact. But moving from that to really being able to accept him loving another are two completely different things. She might very well not be poly, but be "accepting" that this is what he needs to do.
When you don't fully trust your partner, don't feel that they've made you a priority (not the only one, just A priority), and are not getting your needs met-- it is VERY difficult to then be open and loving and accepting of them doing these things for somebody else. And after infidelity (and that's what it was as they weren't poly when he was out running around and lying to her), it takes quite a bit of work and effort to rebuild that trust before the relationship can heal.
And you can "intellectually" think that poly seems to make sense and is what you want, but if you're not emotionally together and healthy you're going to find the hurdles and issues that get pulled out a lot harder to get over.
To me this seems like another case of a couple NOT getting their shit together before dragging somebody else into the mix. And unfortunately, not only does the couple suffer, but the individual that has come into the situation usually gets a lot of drama and/or the short end of the stick. :-/