Discovering that your sister-in-law is pregnant can evoke a range of emotions, from surprise and shock to concern and anxiety. If the pregnancy is unexpected, you may worry about the well-being of your brother, his wife, and the potential impact on your family dynamics.
If we consider a more neutral or less informal way to phrase the initial part, we could say, "Visiting my brother's wife - My sister-in-law..." Knocking up my brother-s wife- My sister-in-law...