She is so regret now

From all the weddings you been too what are some of the stands you remember the most ? Eg:  My cousins there wad hardly enough food.  Friends: it was a fancy place. Open bar Exs cousins: hated the food, awesome bartender Other cousins: good food, cool dj, everyone got kicked out for staying to long. (No one wanted to leave)

I am a bride getting married on September 6th 2020 (I have been engaged for 2 years and have been waiting for this day for a long time). With everything going on, it's possible that our wedding might get postponed until 2021.  Our venue is not ready yet to give us an alternative date yet in case that happens (they will within the next month). In the meantime, both my cousin, who is my MOH and my fiancé's sister, a bridesmaid, got engaged during the pandemic, want to get married in 2021, and are looking at venues already. My fiancé and I politely asked them that under the circumstances, to please refrain from picking a date until we know what our potential alternative date is (we are not stopping them from getting married in 2021). This request was not well-received, particularly by my fiancé's sister and his parents, who believe that she has every right to choose whatever date she would like regardless of our situation.  She is looking at a venue in person today and I'm getting anxious that's she's going to fall in love with it and want to book it ASAP.  I don't know what dates will be available for our wedding, and either one of them picking a date before us limits our already limited options.  It's making already uncertain situation that more anxiety-filled. Any advice, suggestions completely appreciated!