What is a good duet song for my sister and I?

My sister and i have been dancing together since i was 3. She's a senior this year and we are going to have a duet. We are having contemporary dance about her leaving. We want it to be really touching and mature so no disney songs;) But we are having a really hard finding songs that fit our story and not just about two people in love. Maybe a song thats popular and pretty, or a song that a lot of people wouldn't know. Also, a second option would be a piano cover or acoustic version of a normally fast song. Just something different. Thank you!