Planning the whole wedding around moh tentative schedule

Hi all, I’m new to this community - recently engaged less than a month ago and hoping to be married in September of this year. We are planning a small, intimate backyard wedding and are not inviting many people, just our closest friends and family.
My older sister is my maid of honor and my best friend. She is currently attending Veterinary school in the Caribbean (we live in NY). I had initially told her that we would work closely to pick a date that she could attend, given that it is a lot of traveling for her and she does not want to miss classes. After switching the date once already, she asked me today if we can push the whole wedding up to the beginning of August - more than a month earlier; because she said she can’t miss any school whatsoever. I don’t want a summer wedding, and we can’t realistically plan a DIY wedding in that amount of time, but I also don’t want to extend my engagement all the way through next year. Her only guaranteed availability is June-early August, and this is all based on a tentative class schedule which has not been finalized yet. I am not sure what to do because she is so incredibly important to me and I’m worried if I pick a date during the fall, she won’t be able to attend. What do I do?