I'm on both sides here.
Right now, I'm w/ Sue_sue. You don't know what will happen, so don't open up the door of all the 'what-ifs' with her. Honestly, right now, take fertility out of it. She's being rude - she needs to be shut down. I think your DH needs to gently but firmly tell her to stop w/ the quesitons and the pressure. You all will have kids when and if you're ready and not a moment sooner.
Then, if she brings it up again, you both can say "we're not discussing this w/ you", and if necessary - hang up, leave, etc.
However, if down the road you do start to TTC and you do find out there is a problem, then you tell her more. I went through 5 years of IF before I had DS and I do understand wanting to share w/ your family what's going on.
OR if you just out and out decide "no kids" - you can finally tell her that.
But those are 2 doors to just leave closed until one of them becomes reality.
Until then, you need to let her k now that this topic actually isn't any of her business.