Just another one to say that although it will feel impossible, please try not to focus on them and their relationship. As pp have said, they may well be together for a while so the best you can emotionally distance yourself from them the better.
My dad is still with his ow over 20 yrs later. I know it would have been easier for my mum if this wasn't the case. But it's hardly surprising they've lasted because they're both as immoral, ruthless and ambitious as each other. They're the perfect match.
What your h does not have however is a clear conscience, and he will never have one again. That is honestly a weighty thing to live with, even if he is happy in other respects. He has to grow old knowing what he's done to you and his children.
You on the other hand are a good person and you have a clear conscience. That's no small thing. And you will I'm sure in time meet someone who is worthy of you.
My dad is very successful and outwardly happy. But I know his conscience weighs heavier each year. Not least because he knows his children know what he's capable of.
Your h knows enough about right from wrong that even though he may not have acted accordingly, he will always know he did something truly wrong and he will not have peace of mind.
I also echo a pp as to whether he's even been asking asking how the twins are? If he's not, then that lack of interest even at this stage does not bode well and there may well grow a distance between him with his own children. This too will fester away at him in time.
You're doing wonderfully and you are being a rock for your children.