I've never heard of him until just now, but do you know about the Worldcat Website?
It is a database of books, recordings, and other materials available through interlibrary loan.
It seems that poor Siegerl is not as neglected as I thought.
I looked up his name, fully expecting to find nothing except Siegfried as the title and Wagner as the composer, but I was surprised.
On the first page alone, I found a biography, a recording of Siegfried Wagner conducting Richard Wagner, a couple of recordings of excerpts, and a complete opera recording.
http://www.worldcat.org/search?qt=worldcat_org_all&q=siegfried+wagner
In reply to your second question, it would be kinda hard to compare Domenico Scarlatti with Alessandro Scarlatti, since DS specialized in keyboard sonatas and AS specialized in church music.
Also, don't forget the elder and younger Johann Strauss.
Of course, there is also the Bach family. Betcha Johann Sebastian Bach's father was a composer.
The Puccini family didn't maintain its stance quite as long as the Bach family, but it came in a close second. Giacomo Puccini had a direct ancestor, also named Giacomo Puccini, who specialized in operas in oratorios. A few of his compositions are currently in print. One of them is an oratorio about the martyrdom of Saint Valentine.
Which reminds me: happy Valentine's Day!