Oh this is horrible! You would think people would have learned.
I had this a lot. We had four years of trying before I had my DS, and we had been married for ten years before we started trying, and then together seven before that. So you can imagine the expectations!
I tried lots of things, mainly ignoring, counting under my breath, deflecting. Jokes tended to work best, I often deflected to my dog as our baby. But for some people nothing worked until I plucked up the courage to say, I've had a series of miscarriages and I'd rather not discuss it. That did work, though obviously in the case of your SIL you don't trust her enough to say it.
I think Adelfas idea is a good one, getting my husband to have a quiet word with one or two relatives did work.
And I agree that it never ends. Despite the many problems I had, frequent surgeries etc people kept asking me about a sibling for DS!!!! As if we weren't trying....