Info on the real Sullivan brothers
The movie was great, but it included a lot of fiction. For example, the two oldest brothers, Frank and George, both served full four year hitches in the Navy from 1937 through 1941. So they weren't at home in the late 30s to torment their little brother about his girl friend, as shown in the movie. That didn't happen. They didn't even get out of the Navy till the summer of '41, so they weren't at home when their nephew Jimmy was born, as shown in the movie.
Like I said, a lot of fiction.
Here's a link to an official Navy website that has a lot of fascinating information on the Sullivans, including their service records and photographs.
http://www.history.navy.mil/faqs/faq72-1.htm
And, it's not true that their deaths prompted Congress to pass a law against putting brothers on the same ship. That's a popular myth. It's also not an official Navy policy. Putting siblings on the same ship is discouraged, but it's not forbidden. If siblings ask for it they can usually get it, the way the Sullivans did. I use the word "siblings" because it's now possible for a man or woman and his or her sister to serve on the same ship. The times they have changed.
Even though their bodies were never found, The Sullivans have grave-stones at Arlington National Cemetery. Here's another great link, that includes photos of their markers.
http://www.arlingtoncemetery.net/sullivan-brothers.htm