I made sort of a video yearbook my senior year... I didn't spend enough time on it to make it as in depth as I wanted...but, anyway...I used the first part of "The Boys Are Back In Town" by...erm...I forget who it's by, but it's the one that was in Another 48 Hours (not the Thin Lizzy song), "(Don't You) Forget About Me" by Simple Minds...and "It's The End of The World As We Know It" by REM.
I sent out a mass email to my classmates asking for suggestions, a few I remember:
Graduate - Third Eye Blind
Good Riddance - Green Day
1999" - Prince--though I don't suppose that one would work for your sister
Closing Time - Semisonic
and the themes from Friends and Cheers
Oh--and maybe "Here's to the Night" by Eve 6.