I'm still thinking it's best not to reply OP.
You could go on forever with replies that say "I'm not going to change my mind, all the best" and receiving "sorry you feel that way, if you change your mind..." in return before sending "I'm not going to change my mind, all the best" right back at them again ad nauseam. Vicious circle of crazy, nothing good shall come of it.
I also agree with GeorgeDawes, Gluezilla is good for one more appeal to you regardless of you replying to the sister or not now. I don't think she will be able to help herself.
When is the wedding again? Must be very soon now?