I am looking for the names and birthplaces of the siblings of Oswald
Adalbert Sickert (born 1828 ... in Germany, I believe).  He is the
father of painter Walter Sickert who was born in Munich, Germany.  He
is the son of Johann Jurgen Sickert.

According to some webpages Oswald Adalbert Sickert was born in Denmark:
://www.google.co.uk/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q=sickert+oswald+%22danish%2Dborn%22

Sickert family:
http://answers.google.com/answers/threadview?id=366700

One genealogy record gives Altona (near Hamburg) as his place of
birth. Though located in Germany and part of the city of Hamburg
today, Altona was still Danish territory in 1828. That would have made
Sickert a Danish subject, but of German nationality.

Scriptor

Ktaylor8-ga, do you know if the siblings were older than Oswald
Adalbert, or younger? Do you know their names?

Oswald Adalbert, born in Altona:
http://www.fada.com/browse_by_essay.html?essay=690&gallery_no=30&artist_no=4886
http://www.casebook.org/dissertations/dst-artofmurder.html

I haven't been able to find any reference to siblings, so I don't know
for a fact that there are any, I just assume.  I think you're right
about Denmark as the birthplace, so any siblings would probably have
been born there also.  Thanks for the input thus far ... kt.