Ha! These responses are funny. I come from a culture that also has no boundaries when it comes to personal issues, and believe me the "polite responses" or evasions will not work. We all know that even in "white" North American culture, people are very persistent on the child issue, and you can forget it in any ethnic or religious culture.
Dolyn, you are right to be concerned, it's going to be very difficult to conceal the truth. I recently had a half-hour discussion with an auntie about being childfree, and it took a lot of humour on my part to be able to parry all her pointed questions; and this is a regular occurrence for me at most extended family events. I can see why your husband would want to avoid this kind of third-degree, but there's no getting around it. You can indeed start off as Chi suggested, since it is true that you physically cannot have children. But eventually it's going to come out that it is a conscious choice, especially since your sister-in-law once offered surrogacy as an option! Maybe you should have a chat with your husband about "signals" you can use to indicate to each other when you're embroiled in an attack from a relative. Then he can come over to help change the subject or try to evade it. I highly suggest witty responses to the questions, they are more fun than serious ones.