Cassie is the daughter of Zeus and a mortal archeologist.
Donna is ... more complicated.
The original Wonder Girl was a flashback series about a young Wonder Woman- just like the original Superboy was a flashback series about a teenage superman. The Silver Age being what it was, there were frequent crossover stories with adult and teenage Diana. Think "All New X-Men" but sixty years ago, and for the purpose of deflecting lesbian/bondage rumors. The writer of the first Teen Titans stories saw these covers and thought Wonder Woman had a sidekick. Comics editorial, being as much a joke then as it is now, didn't catch the error until it saw print. This then left them with the challenge to create a backstory for Wonder Girl.
Donna Troy changes origin, codename, costume, and powers as often as most people change their underwear. I'm just going to copy-paste from wikipedia for this part:
http://en.wikipedia.org/wiki/Wonder_Girl#Multiple_origins
Like I said earlier- no matter how much they retcon and reboot, Hawkman and Donna Troy will never make sense.
Same mother/sculptor. Or adopted sister. Again, depends on the origin. Although in the New 52, Wonder Woman was made the old fashioned way, as the product of a one night stand between Hippolyta and Zeus.