One-day,
I think a lot of your anger stems from the general view of marriage and family by your "FIL", and partly your MIL as well.
He fathered your H in an affair, which tells you he didn't have much respect for marriage.
He never married your MIL.
He never had much to do your H as he was growing up.
He returns to the life of your H, and tries to take on the role of "father", and your MIL actually LETS HIM BACK???? Wow. What does that say about HER values?
He has what you call an "affair", which, how can that be when he isn't married to your MIL....but that shows his "committment" to marriage and family even more, doesn't it?
And now, there's another "other woman's child" in his life. I suppose that his first wife is not too surprised by his behavior. And your MIL should not be at all surprised.
So, I can see why you are angry, given that you are fighting the demon of adultery in your own marriage.
Your husband did not have much in the way of role models in his life. The value system set before him wasn't terrific. What he was shown was that fathers can step into and out of the lives of their wives and children whenever they want, and be accepted. Women are usable, children are adaptable, and men can do what they want.
Only now, your husband has one thing different in his life:
YOU.
And you can make all the difference. Remember that.
Tell your husband that you are angry. Tell him that the value system makes you mad.
Tell him that you need to know about what he values.
And when he tells you, and shows you, that he values you and your kids above the rest, you will feel safer about this. Right now, the anger comes from the fear that your husband has been taught this value system. And the fear is justified.
Only you CAN be the difference - because you ARE the difference.
The woman behind the man.
Be a team WITH him - talk to him about the anger and the fear - to do otherwise is a mistake.
SB