Can I warn my wife?

So I just found out my brother-in-laws girlfriend is pregnant. My wife and I have had our doubts about this girl and we have feared she would get pregnant to trap my brother-in-law in the relationship. I'm worried if my brother-in-law tries to surprise my wife with the news she won't act excited or supportive. Should I keep it a secret or warn my wife?

On a side note, we have been TTC our second child for 6 months now...