I can't speak for the op but from what I have read, I think she will give her marriage a chance and so will her husband. They'll work something out fairly regarding his other child. Nobody knows what will happen in the future but they may be OK in a few years, or maybe not - nobody has a crystal ball. The other woman will probably meet someone and marry in time. Time heals!
Somebody earlier, name beginning with H, gave what I thought was some very wise advice.
It's difficult for all of us because we do not know the op, her husband and children or the woman with whom he had the brief liaison. We're all human beings and make mistakes. It's unfortunate that an innocent child is the result of his mistake but as long as he/she is cared for, life won't be too bad for them.
I mentioned earlier that I knew two couples who were in the same situation. They managed to weather the storm, one couple are still together after forty years of marriage, their children were told they had a half sibling (conceived during a break), when they were about 11 and 12 and they get together regularly. The half sibling's mother married someone and all animosity is long gone.
The other couple broke up some 25 years after the baby was born but it wasn't because of that, they had grown apart and the wife remarried fairly quickly. They had two children who knew nothing about their half sister (whom 'dad' had always supported and saw regularly), until they were grown up so it was a bit of a shock but they became friends.
Very little in life is insurmountable or unforgivable.
I wish the op every success and happiness in the future.