Kimmy,

I live in the Southern Hemisphere, so on my wedding day in August, there was always a chance of rain.

In the morning, it threatened, then cleared enough for me to walk OUT IN THE OPEN, did the ceremony, tooks pics, ........... and then it bucketed down for about 4 hours

So, fingers crossed for you!!!! I am sure it will be WONDERFUL no matter what.
And I LOVED your sister's cake!