#1 - You can't change her. Obviously, her ranting drove her son to the place he is now regarding not wanting children. You are definitely insightful to see her own longing and loss mixed up in all of this wishful ranting. Let her be. How sad that she has chosen this fantasy as the one item missing in her own life to make it all complete. But she needs your uterus to complete it.
#2 - Be as sweet as pie and lie through your teeth. Just tell her you both want children too one day and you'll be trying and it's all in the good Lord's hands and His timing and you don't want to hear another word from her about the topic. There's enough pressure and stress adjusting to an instant family, yada, yada...and we don't need you too..so you better call before you come over. I love Lesley's 2nd paragraph....what is she going to do , steal your birth control? Poke holes in condoms? Just make sure to hide your birth control so that she doesn't ever catch on.
#3 If you were to go to your family doctor about wanting to become pregnant, they would tell you to try naturally for at least 1 year before they would consider any type of fertility testing. Largely due to your young age. So if she does pester you, that line will buy you at least a year. But honestly, even though there are lots of moms telling your husband to step up to the plate on this one...it sounds like he has tried and is just plain tired of the topic as well. You MIL is going to drive people away if she continues on with this broken record message.
And finally, there are many people who honest to goodness don't get that they are being a pest until they are directly told so to their face. She sounds like this type. I know at 23 it sounds awfully hard, but this isprobably the route your will inevitably have to take with her. You will have to kindly and firmly look her in the eye and tell her...we too want children one day....when it's right for us, not for you.
And like many other moms here, I had my last baby at the ripe age of 41+ . We are fertile for a very long time. Enjoy your new marriage and your new family.