Abby and Martha Brewster were played by Josephine Hull and Jean Adair, the same actresses who played those characters on Broadway. Jonathan Brewster was played by Boris Karloff in the play, but he couldn't get time off to make the movie, so his part was filled by Raymond Massey.
All of the casting is pretty much pitch perfect. Even minor characters such as Mr. Witherspoon give memorable performances (Witherspoon is played by Edward Everett Horton whom I immediately recognized from a number of Fred Astaire films).
This is, as I mentioned, my family's favorite Halloween movie, being appropriate for all ages, a little bit scary in places, and a whole lot funny throughout. It's one of those films that I can see every year and not get tired of. Even knowing full well all of the jokes and plot points, Arsenic and Old Lace still makes me laugh every single time. I love this film.