but surely you have the torn half of that picture of your sister… and she kept the half with your picture.
In the wake of new techology today, you both will simply post your two bits on Flickr 
and you’ll know.
Or… wasn’t there a toy gun you hid under the sand in your childhood… and when you were visiting a place near your childhood home, you get a burst of nostalgia and go digging for it. As luck would have it, your long lost sister was also in the area at that instant and sees you retrieve it… you embrace each other… and loud sobs of “bhaiyya”…and “behna” ensue.
so true… and for anyone who wants to know… the movie was Desh Premi… sincerely… Encyclopedia Bachanica.