Salute.

So you fucked your wife’s sister and knocked her up, permanently shattering their family relationship the second your wife finds out, and you’re seriously here to whine for sympathy? Dude.

you're a dumb ass

Stick around and be ready to be called "Uncle Daddy." Living with your mistake is probably the most fitting punishment.

What is wrong with you!?

So your children are siblings and cousins at the same time. I'm sure they will grow up just fine.

convert to Mormonism

It's time to 'fess up. You're not going to get anywhere by not telling anyone--confess to both of them, apologize, and prepare for the ensuing shitstorm. Do what it takes to make it up to them both, and be prepared to step up for both children. You fucked up epically, so it's time to admit it, apologize, and do what you can to make it better. No other way forward in this life.

They have better be twin sisters.

Why the hell would you be in that kind of a situation with your sister-in-law at all, let alone get her pregnant.