I guess I mean that I'm going to have to let them know I'm not game for sex with the two of them. I've already told my husband. He was disappointed and kind of sad, but glad I was honest so we all know where we stand.
You bring up another good point - I am not interested in her emotionally either (nothing beyond being respectful and polite). Their relationship has been professional for many years and emotional for most of them, romantic once a couple of years ago when he and I were on very rough terrain, and now we've decided as three adults to allow them to indulge that emotional and physical connection openly.
I was invited into their relationship. I just don't feel connected to her, though I did give it a really good shot. I guess part of me is disappointed, too, that our first poly relationship attempt resulted in me not feeling connected - but I certainly don't blame the two of them. It's just not there.
Husband is worried that it will be awkward now. Maybe I just say that we can get together during weekend days for fun with our kids (they adore her) but sexual sessions will need to be confined to her house.