If she is having marital problems, the LAST thing she needs is to bring a child into this! Her focus should be on their relationship. A child WILL NOT FIX THAT! When the time comes to tell her, and you'll know, just remind her that she does NOT want to bring a child into a troubled marriage. She can't focus on both and give either one what it needs and deserves.
Remind her that you'll be there to listen, etc when she needs it. If she wants to come visit and play with your kids, set aside a weekly visit or bi-monthly visit for that. That may be some of the best "therapy" she could get. I know having a toddler around certainly helped take the stress off some of the things I was facing the past couple of years. A differenct situation, but "therapy" none the less. Plan projects, visits to museums, etc. for all of you to go. This could be a stress reliever for her, too.
If it turns out that you do wait and she ends up noticing, then just tell her that you are more concerned about her and her marriage. This is unselfish and not withholding. Sometimes, it's not appropriate to just say, "OH, BY THE WAY.......". You'll know when it's the right time. Don't force it. Concentrate on her right now. If you think there's a chance someone else might tell her, you need to start a conversation by saying, "I want to listen to what you're feeling and dealing with right now, but I you need to know this and I want to make sure you hear it from me." Remind her again that you haven't told her until now because she needs someone to listen and have been more concerned about her situation.