In our Guidelines we explain that it is important/required to be gentle with newbies. Accusing her husband of cheating seems harsh and unwelcoming. He came to her and confessed he had feelings for another woman. Catching feelings for someone else and admitting them to your spouse is
not cheating.
This forum was founded on the idea that communication, empathy, and the functional relationship skills so necessary for poly relationships should be fostered here. The primary rule of this board is for members to be considerate towards each other. Other guidelines for board members include (but...
polyamory.com
I also don't see where the OP said they were currently in a monogamous marriage. She said they were opening up.
Serenity, you are not suddenly in a "relationship of three." You are in what we call a "V." Your husband is the "hinge." You and the new person are the arms of the V.
(You might want to choose nicknames for your husband and his new dating interest person, for ease of reading and getting feedback.)
What this new V looks like is up to the three of you. For example:
You don't have to have her over often, spending time with your children, for one thing. In fact, many people with new partners don't introduce them to their kids for a good long while. If the kids establish a bond, but the new partner doesn't work out, and suddenly stops coming around, they might be sad and upset and ask uncomfortable questions.
You personally don't need to hang out with the new person often, or even at all. You don't have to have a deep relationship with her just because your husband feels he loves her. If you only meet her in passing at the door, being polite, that is fine. We have a term for it: "parallel polyamory." If you see her occasionally on special occasions, such as birthdays and holidays, we call that "garden-party poly." If you three all hang out often, getting along great, it's called "kitchen-table poly." Finally, if you and she begin to be sexually attracted to each other, or romantic, and decide to have sex, your V becomes a "triad." (Some people call this a "throuple," but I personally don't like that made-up word.) You are in no way required to date this woman.
In fact, if you get your head wrapped around polyamory, and you continue to open your heart, you might find yourself attracted to a new person too, start dating another person for yourself, independent of the relationship between husband and New Woman. And this person might be female, male or some other gender you might be attracted to. Don't let your husband assume polyamory means you and he "share" this one woman! That is not fair. You might not even enjoy a close platonic friendship with her.
Please check out our Golden Nuggets section for links to many articles on polyamory, as well as a list of books, a helpful podcast, and links to our own archived consolidated threads on every aspect of polyamory imaginable (going back to 2009, hundreds of threads!).
You might start with the books
Opening Up, and
Designer Relationships. Polysecure is also very popular and helpful.