This is a horrible situation. However, the husband needs to know i am sorry if this does not make me very popular, but it is going to crush him though a secret this big cannot be hidden forever and it should not be. I cannot believe this woman im sorry but she is a piece of work and that is coming from me.
I am sad for you as you have put up with this for too long and should have walked away along time ago, but you did not and unfortunately actions have consequences and she needs to face the consequences and you will face an uphill battle to as the fall out sucks.
The husband needs to get DNA tests to find out if the kids are his or yours.
The children are the true victims of this, but they need to know too as it will hurt later in life, though i imagine it is going to hurt like hell anyway.
i think this is one of those times when your dammed if you do and dammed if you dont.
Apart of me wants to tell you to walk away from this woman and let her suffer, because there is too many that will suffer as this is not your typical situation as the children have grown up thinking this guy is there dad.
Do what you think is best for your children and for you.