what do i tell my siblings... sorry it's so long

so my wife and I determined the week after we got married that this December we would start trying and then a few months after we got married my older sister (2 children) got engaged and set her date for July 2018. While getting fitted for my bridesmaid gown I told the lady I was nervous about the fitting because I was either going to be much skinnier or very pregnant. My sister responded with you will not be pregnant at my wedding. Then a week ago my sister's made a plan to get sister tattoos in January. Basically, if I get pregnant next month all of my sister's are going to be pissed at me. I keep telling myself how is it different if one of the three of them got accidentally knocked up by their significant other. How do I over come this.