I don't want my sister to schedule her wedding a week before mine.
Why don't you want your weddings so close together?
Because I'm worried that her wedding will ruin mine.
It's not reasonable for you to try to dictate when your sister has her wedding, but it's understandable that you don't want your wedding so close to hers. Weddings impose a financial burden on families and guests, and if you have a sister who's getting married around the same time as you it may make it more difficult for people to attend. Try presenting this idea to your sister and see if the two of you can come up with a solution to make things easier on everyone, including yourselves.
I need to schedule mine first.
I could somewhat understand if you want to be married first, perhaps, if you are the older sibling.  However, you shouldn't tell your sister when to get married.  This particular date might have a special meaning for her, so at least try to work around her desires.