I would say, take it slow, give yourself time, don't rush into ANYTHING, don't feel like you have to be "normal" or any kind of way for anybody whether it's him or your sister or whoever. Just be very gentle with yourself, you have been through a great shock as others here say. It's true. It pulls the rug out from under you.
As for him, it takes time to trust again, if you can. Don't beat yourself up either way, just realize it takes time and you have to see consistent effort from him. Consistent behavior, consistent effort. I think it helps - it might for me - to know why he did this and why he would not do this again. That would be my question - not just are you trying to placate me to keep us together, but....what have you learned from this? How does this fit into your plan of what you want from life? And that's a question for YOU too. This is a good time to assess what YOU want as well. You have to be assured that he really has changed on some level and that he has learned from this that HE does not want to do it again because......because why? He has to answer that. That's what I'd want to hear. And as I say, I would not want it to be just to make everything sweet at home again, but to see some real change and insight from him. Especially about your feelings. Not matter what though, it takes time, and expect to have occasional triggers because I think we all do. If all is going well and he is meeting your needs, expectations, seems to have learned, etc, you can learn to let the triggers pass.
I would not plan for marriage for any time soon, frankly, I'd talk to a lawyer before getting married and talk about the whole situation and see what he recommends for your protection and any children you have (I forget now if you do). He might recommend a pre-nup or some kind of arrangement to give you extra protection perhaps beyond the law so you have more security in a marriage with him. I would also figure out what you want to do long range on your own, why you personally want to achieve and how you can make a living on your own, either now or in the future, with kids because one never knows. It's always good to be able to bring in your own money and have independence just in case.
As for the OW. It's absolutely terrible when a friend or relative does this, I think it's unforgivable because in some ways that's a deeper relationship than even a marriage. A relative is blood. I think this would create more unpleasantness for you if you confront her or seek a confrontation. What will it do for you? What would it accomplish? I am not against confrontations, hell, I am not against fist fights, but I want you to do what's best for YOU. Would confronting her keep her in your life more....or would it help to cut her out? Are you just so angry you don't know what to do with it? Think about what this would accomplish for you and what the ripple effects might be as well. Maybe it's worth it for you, maybe not. My guess is she probably would put on a bold face and just be nasty to you, and you'd leave angrier than when you started. I'd probably pass up on this if it were me - unless he pushed herself back in my life for some reason. But if she's staying out....I think I'd keep her out and work on doing anger exercises - like the letter writing is a good one. I like boxing or breaking stuff ( in a safe way of course) that might get out physical anger....emotions are IN the body too, not just in the mind. Maybe you need to work it out in your body.
As for your sister - give her a big old mulligan. That means overlook. She's angry for you and I would be pleased to see it if it were me. I think it shows how much she cares about you and wants to protect you. I love that she's so angry with him. People SHOULD be angry with him!!!! I think you might be displacing anger you have at him, because you want to reconcile, towards her. I would not do that. Have a discussion with her about this and let her express how she feels and tell her you'd like a better atmosphere when she comes to visit. She can put on a face anyway. It's gonna take a while before she changes towards him because....frankly, he deserves the anger and she's right. She may need your help in learning how to handle it in a way that makes YOU more comfortable and that fits in with your goals. But don't reject her opinion either. A good sister is worth gold.