I don't think going on this trip makes you a bad mom at all. Absolutely not. Hopefully your daughter continues to improve and it won't be like she'll be all alone with your eldest being around. Like you said, you can't do the healing for her. Plus her being an adult is different than if it were a young child. Besides, wasn't she surprised that you were contemplating canceling?
Ultimately you should do what feels right to you, but you don't have to feel guilty for wanting to go on a trip you've been planning for a long time.