What (not) to Wear

My sister is coming down for the weekend to see her boyfriend and she texted me yesterday that they are attending a benefit Saturday night for a non-profit that her boyfriend's mom is on the board of. Her boyfriend "bought" a table (or however that works) so they invited me to join them for the evening which, don't get me wrong, I'm psyched about! However, I need to find something fabulous to wear in 24 hours. It's black-tie optional. Do you think I should go long or short with the dress? I have a black dress that would be somewhat appropriate but back is so boring...

My sister's boyfriend's mom is wearing a silk skirt-suit and he and his dad are wearing suits. She's arriving around 5pm today and I'm guessing first on our to-do list will be to find dresses...

Regardless of the outfit- - I am SO excited for tomorrow night. The first line of the invitiation reads, "Put on your dancing shoes..." Put them on? I was BORN with them on. Woohoo!