So sorry you are going through this! I give you major points for setting a boundary with your family (arrange to meet in another town) - that's hard to do.

PTSD is no joke - you know your limits right now, and your triggers, and are strong to stay firm on this. You are very caring of your extended family, and cautious of their feelings, but your health and functioning are also a priority for you, your husband and son. I see you love your sister, but an not sure it is fair to announce an intention to visit without asking the host's approval, first. You don't sound impulsive, and were kinder and more measured in your response than I might be.

One month of work, even with a great therapist, probably won't provide much of a buffer against the family dynamic, in close quarters, for an extended visit. Pardon for asking, but will your husband back up your wishes on alternate arrangements?

Stand firm; you would not be hurting them by taking time to get healthy/feel more mentally secure, and what benefit for your son to see his mother continuing to protect her health.

Take care! - Bolivar