You won't know, because your wife will have already killed you.
Don't think there's a word for your spouse's illegitimate children.
what does the child call your wife, step-sister or step-mom?
:hmm:
Mother-in-law would have to be pretty young to get pregnant. Which means you either married a 13 y/o or she had children at a really young age, or both.
Sounds like an episode of Jerry Springer.