Previous affair, from a previous marriage is claiming to have kids by me. Help !

Job One: insist on DNA testing.
Job Two: Get a lawyer. Learn what you will need to do if the roll of the dice comes up as baby daddy for at least one of them.
Job Three: Talk to your wife. Explain there is a possibility but you are checking and that you will do the right thing if required to, as the child(ren) is/are innocent.

Do all of these things at more or less the same time. That is, don't leave your wife hanging while you wait for the results of a DNA test or for a lawyer to call you back. Tell your wife so she's not blindsided if it turns out you are the daddy and even if you're not, so she knows what you have been dealing with emotionally.

First you get paternity tests done.

Just so blindsided by all this....

Thank you for the advice so far.

Just wondering how my wife will take this news...we have a great relationship and a baby on the way

Your wife will take it best if you let her in on it right now. The longer you wait the more she will resent being treated that way.

Could you explain how she has TWO children with her that you claim you didnt know about?

Twins

What else do you know about these 2 children? Do the children have your name? (Last name)?