The MC you two attended a while back is totally different. That was for the two of you, and she was probably not in a place where she was truly interested in making the M work.
Family therapy now is entirely different. You say you organized it and she didn't know about it. Why didn't you get her input before you signed everyone up and scheduled it? Do you guys have any formal parenting agreement in place? In my D there is verbiage that basically says that things like doctors, dental work, extra curricular activities, etc, all require both parents on board. It's odd to me that you'd do this on your own and then think she should want to participate in any way.
So if I were you I'd tell her that you felt it would be good for the children to attend therapy, but that they are her children too and if she had concerns to let you know. Then if she was ok with it, I'd mention the IC thought her participation could be beneficial and extend the invite for the best interest of the kids.
However it plays out it's important to keep in mind that from a DB perspective the only thing that has anything to do with DB is how you choose to co-parent and how you choose to treat her. If there's any trace of wanting some type of family time with her, wanting her to talk to a therapist under the guise of family therapy, or anything to do with hoping this could help restore the marriage, then I think it's a bad plan because your expectations will do more harm than good. But if your focus is on what's best for your children solely then share that with her, and get her feedback both on therapy and her attendance so you two can start co-parenting cooperatively.
Me:38 XW:38 T:11 years M:8 years Kids: S14, D11, D7 BD/Move out day: 6/17/14, D final Dec 15