Pause your game and make a backup copy of the save game file, just in case they are not successful and don't get to adopt. More info in this thread.

If she's already 48, you have between 8 and 12 hours until she hits 51. (There have been a couple of players that reported babies after that, but we haven't seen any screenshots.) Just keep trying as often as they are willing. After the female is age 50, you still have a chance of getting an adoption event. So, it is up to you whether you choose to reload your saved game and start again at the current age, or wait until after one has passed away.

If they don't have any children to pass the house to, that family will end and you will have to start over.

If you need to know how to backup game files, see this post.

Good luck!

Edit: Ohhh! I AM a blonde. I'm so sorry - It's corrected now. Thanks for the pm.