I'd assume they are NTSC quality if they're on a DVD that plays in a DVD player. Why not contact your cousin and have her send you the photos? Video on a DVD would be VGA-like quality which isn't very good...
If you still need to extract the "photos," you can't take screenshots via Apple's utility but I think other screenshot utilities will allow that. I'd probably rip the DVD to my computer (via Handbrake @ High Profile-->Film) at it's highest settings) then show the video in QuickTime. Play it fullscreen and pause it on each picture and hit command-shift-3 to take screenshots.
I'm sure someone has a better way, but regardless of which method you use to extract the pictures from the slideshow, they'll be low resolution.