I understand this is very hard. It would be good for the sister's to meet. I mean, it could be a very important relationship for your daughter.

It may be painful for you to go. But, it could also be theraputic as well. Sometimes we have to let go of the things that hurt us and try to move forward. Instead of seeing this woman as "the b*** who slept with my ex," see her as "the mother of my daughter's sister." I do think it is unfair to request that the mom not be there. It will be awkward, for sure. But learning to let go of things that hurt will help you and your daughter in the long run.

For example, my ex has full custody of my son. I would rather never see him again, but that's not possible. A few months ago, my son had to have surgery. I had to spend a whole morning at the hospital with my ex, his fiance, and my ex-mother-in-law (who was always hateful and judgemental even before the divorce.) Was it comfrotable? No. Did I get through it okay? Yes. I was there for my son. It didn't matter who else was there, I know he needed me there, too. I actually like my ex's fiance (in truth I feel sorry for her, knowing the type of person he really is, LOL,) and so we had a nice conversation.

And in the end, my son knows he has support on all sides, which is the most important thing.

It will get easier with time. Don't let your daughter miss out on getting to know her sister.