I need some help finding some comedy movie clips (5-10 seconds) with a few specific themes:
Brothers or siblings, maybe fighting, older brother antagonizing younger brother, etc.
Grandparents
Extended family
Parents and kids
Thanks for any advice!

I'd start at YouTube.

Here's a good one... from YouTube

YouTube - 4 laughing babies (http://www.youtube.com/watch?v=yE6PNps5N9I)

Sorry, I should have been more specific. I am really looking for clips from real movies that will be recognized by either a 40-something generation. Thanks!

Have you looked up Marx Brother clips? YouTube - marx brothers (http://www.youtube.com/results?search_query=marx+brothers&aq=0)

Then there are the Three Stooges: YouTube - Three stooges clips (http://www.youtube.com/results?search_query=Three+stooges+clips&aq=f)

Choose a movie and look for clips on YouTube.

Who's your audience?

Here's a suggestion

Weird Science
the best of Chet (http://www.youtube.com/watch?v=kAH4vipYD_U)