I'd wait a bit until after she has the baby to get a decision from her. Her attitude about what she can and can't handle might change. Even though this is baby #2, it's not going to be the same as baby #1. She might decide to leave the baby with
her husband and savor a couple of days away. Or, she may feel like the can't be apart from the baby for that long and opt to not come at all. It sounds like she has her heart set on being there to see you get married. I'd give her a little slack
there, as it seems to be really important to her.
Traveling with a newborn is actually a lot easier than traveling with a toddler IMO. Ideally, they sleep the whole way. And a flight from Cincinnati to Dallas isn't that long of a flight, (less than 2 hours?). But newborns are still
developing their immune systems and flying exposes them to lots of new germs. Some moms aren't cool with that, some are fine with it.
If you're pretty close to her husband, how about talking to him about it? Maybe he can convince his wife that he'd be happy to keep the kids for a couple of days while she has a mini-vacation. If that doesn't work, talk to her about how you dont
want her to miss out on any part of the ceremony if she has to leave with a crying baby. You want her to enjoy herself and not have to worry about keeping the baby occupied and entertained.
Good luck!