OK, so let's look at this logically, from all angles.
Your DP and this woman had an emotional affair. It crossed several lines. while we don't need to have the details, are you sure this wasn't more than an emotional affair? there seems to be a lot of emotion running very high and a great deal of contrition/shoulder shrugging from your DP, perhaps to get this matter left alone?
Your entire family won't have anything to do with this OW, and DP entire family aren't impressed either.
The ONLY ones who see no wrong are your sister, and presumably this OW.
Does your sister not like your DP? is she jealous of who he is/what he has? or is she jealous of you? does she compare herself to you? Is there a reason why she would deliberately keep someone in her life to be a thorn in your side/DP side? what are her motives for putting up with this amount of flack?
You say she has a good sense of right and wrong... how will she react when OW makes a play or encourages a fumble with HER DP? she doesn't resepct the normal confines of relationships. Yes your DP has a role in the EA, but so does she. She could have stopped things just as easily as he did, because of her friendships with you and your sister if nothing else.
How YOU feel is important. Vital actually. the feelings of others in this instance is NOT as important.
Yes, you read right, your feelings on this matter MORE than the OW, and MORE than your sister's tbh.
I do understand your DP stance of supporting whatever YOU want to do in this, but you feel you could do with him saying 'We'll do this" and then all you have to do is say, "yes, that's what we'll do"
But you are the wronged party here, he has a duty to put you first and back you up, to rebuild the trust and solidarity he lost when he conducted this EA. Your family support you, his family support you, which is fabulous, but yes my dear how this plays out is very much YOUR call.
If you want to be absolutely true to yourself and honest about your stance/wishes, then you do need to be very clear about what you can and can't do. Actually, you also need to NOT worry about what OTHERS can or can't do.
You have said you can't do the Hen do. that's absolutely your call to make
You have said to us that you don't want to be at the wedding if she is there - you need to tell your sister that and push this issue of her making back on her.
The OW needs to back out. Your sister needs to manage that, but you don't have to commit or agree to attend anything UNTIL she does so.