If you are looking for a way to make a sister-in-law who is not actually in the wedding, but will inevitably be hanging around, feel included, I would start with extending an invitation to her for her to ride on the Party Bus with you and all of your bridesmaids.
The bridal party for the wedding was too big for a standard limo, so they rented a party bus. I had never been on a party bus before, but was not picturing a giant limo. I was picturing something a little fancier than a school bus, kind of like when my family chartered a bus for the Renaissance Festival for my grandpa's 80th birthday. Instead, the party bus pulled up complete with flashing lights and a soundtrack that started off with Going to the Chapel. If you have to travel to your wedding, I highly recommend the party bus.
A quick tip though - make sure you tell your limo driver any shortcuts you usually use. Ours made us about 10 minutes late to the ceremony by taking the major roads instead of the back roads that my in-laws use to get to their church, and since the windows were down, we didn't realize where we were going until it was too late.