DoubleWhammy,
I hardly ever come to this site anymore and rarely ever comment, but your post and the many responses you've received somehow really got to me. And I'm many, many years past my first d-day and a lot of years past the one the finally ended my marriage.
First--you seem like a lovely and articulate woman and I'm very sorry that two people you love so much hurt you like this. Frankly, the fact that you are even able to construct a coherent sentence this soon after what happened, much less explain your story and respond so clearly, is a pretty good indication that you are a tough cookie. Hang on to that--you're going to need it.
The single best piece of advice I got when I went through this was from my therapist who simply said, "don't be in a big rush to get divorced; you can ALWAYS do that so don't make that your first decision or think that's the only one you can make." I ultimately did, but not for a long time. I waited for changes in behavior, remorse, ability to trust and indications that under the cheating and lying was a better husband and father. When it was obvious that wasn't going to happen, I did what was necessary to make sure I could support myself and my kids, and then divorced when I was ready.
It sounds like your husband is doing all the right things, answering questions, treating you better and that's promising. But I also promise you this, at some point he will get tired of it. Especially on those days that the roller coaster he has set in motion makes you sad and angry, when you get triggered and lose it and when you let him know just how horribly what he did hurt you. He will accuse you of not being able to "just get over it, for God's sake. I'm doing everything I can and you won't let it go."
Then he'll start in on what you did to help this happen. Like he tried to do before you knew what he did with your sister. So be ready.
And don't get me started on your sister. I have three daughters and if I found out that one of them had done something like that to one of the others I think I would choke her with my bare hands. But I digress. . .
You need to make the decision that works for you, your kids and your family. By the time I divorced my ex I felt it was what I needed to do. It still wasn't easy. Not then, and sometimes still not now. Every family thing is different due to divorce. Birthdays, holidays, graduations, weddings, funerals.
I hate missing any time with my kids, even now as they're adults. I hate that their father has a new wife who is threatened by my girls which makes her mean to them, and two little boys that my grown children have to figure out how to deal with and how to explain to people because their father and his wife are old enough to be the kids' grandparents.
And I also envy a close friend of mine, who found out years later that the husband she adores cheated on her with a co-worker. He's been a perfect husband since, but it took her years to deal with the shit sandwich she was served. She chose not to tell her kids or anyone in her family so they could continue to see him as a great guy. Now they've recovered and are happy as can be. They have a great time doing fun things together, travel and are each other's best friends.
So I guess what I'm saying is that there are lots of ways to do this. Only you know what can work for you in your situation. It just sucks that it takes such a long time to figure it out and that it is so painful while you do it.
Good luck to you, strength, peace, healing--all of that and more to see you through. Take care of you.