You have to take care of yourself and sometimes that means setting stronger boundaries. It is rude of your sister to plan your summer visits.
You extended a great deal of courtesy letting her know your husband would meet them. Good for you in standing up for yourself. It will get easier.

Edited to say~ yes, I have dealt with this many years ago but I didn't waste my time with guilt. I didn't have the energy and it sounds like you have a limited supply as well.