Two sisters weddings

Hi all... Over the last couple of months, I have gotten engaged and so has my little sister. We are very excited, but unfortunately we are both wanting to have our weddings in summer 2018. My sister is pretty set on June 2018.

Im wondering what time gap we should consider to have between the weddings. We both like warm weather and are having outdoor weddings. Any ideas or suggestions? Not sure if this is a big deal of not but I do wonder if it's something we need to think about when setting dates.