A new trend at weddings is the brother/sister dance, often because the brother is taking the place of the "father." Picking the right song can be tricky because a lot of songs could be...well, creepy.  These could work:

Brothers and Sisters - Coldplay

Dance Little Sister - Terrence Trent D'Arby

We Are Family - Sister Sledge

When We Grow Up - Diana Ross

It Don't Have To Change - John Legend