I get where you are coming from too... My sister and my SIL have been trying to conceive for years... Like 3 or 4 years each. My sister has had two miscarriages and for whatever reason, my SIL can't conceive at all thus far.
I am now PG with our #3. DD is 8 and DS is 7. I "warned" both of them this summer that we would be trying hoping to not shock them so bad. I told my sister a few weeks back, and luckily, she is PG and is having a very healthy pregnancy. We are very thankful. My SIL, however, I have not told. If we don't, she will be blindsided at Christmas, although she did have the "warning" this summer.
So, if you figure out what to do, let me know. Very hard situation. =/